Depends on how you define decay,
At the time of Lancaster's death, the nun's body was not embalmed before her burial and her casket was made of simple wood without an exterior layer. Workers expected to uncover boned but instead found a perfectly intact corpse. After seeing Lancaster's intact body, the convent's Abbess, Mother Cecilia, described the actions the nuns had taken, according to Newsweek. 'You can't Google, 'What do you do with an incorrupt body?'' she said, 'so we started with the basics, just cleaning her with hot water, because clinging to her face was basically a mask of thick mold.'
And from https://people.howstuffworks.com/incorruptible.htm mummification can also happen accidentally. In the 19th century, it was customary for the dead to remain interred for five years before being exhumed and cremated. And in 1865, when the first residents of Guanajuato, Mexico, were exhumed, the town was in for a surprise. Undertakers discovered that unembalmed corpses buried in the town graveyard weren't decomposing -- they were mummified. The salty, dry soil in the graveyard is responsible for the mummies now on display at the famous museum in town [source: Ball]. Another case of an incidental mummy is the Tollund Man, a 2,000-year-old prehistoric man. He was hanged in Denmark, and in 1950, his body was discovered -- remarkably preserved from the peat bog into which it had been dumped. Even his fine hair and beard are intact [source: Silkeborg Public Library].
These examples of preserved humans share a common theme: Science can point to the means (either intentional or environmental) by which they were preserved. This is not the case for some of the incorruptibles found around the world -- their existence baffles scientists. While the preserved remains of mummies are generally found in states of rigor mortis-like petrifaction, incorruptible corpses are pretty pliable. Their skin is supple, even years after their deaths. They appear, for all intents and purposes, to be sleeping or only recently dead.What's more, these corpses don't show signs of having been embalmed. And the local conditions don't appear to have had a preservative effect on them. While they remained in a perfect state of composition, other corpses interred nearby were degenerating like normal. ...And at least one case of incorruptibility was discovered in a person who clearly hadn't exactly lived a saintly life. Cardinal Shuster, an Italian archbishop, had been a fascist and friend of dictator Benito Mussolini. His corpse was found uncorrupted 31 years after his death [source: Fortean Times]. - https://people.howstuffworks.com/incorruptible.htm
But there can be something about preservation:
Sixty years later, during beatification proceedings, her [Catherine Laboure] body was exhumed. She appeared alive. The corpse was treated with chemicals and put on display. Today it lies in the Chapel of Our Lady of the Miraculous Medal in Paris. (https://www.evl.uic.edu/landry/bodies/cath.html) Likewise, "The body of nineteenth-century visionary Catherine Laboure was found incorrupt in 1933 and was maintained by artificial means." (Christine Quigle, "The Corpse: A History" p. 256)
Vatican’s secret, and deadly, project to mummify saints
With Ancient Egypt’s mummification process as inspiration, the Vatican had an elite team of embalmers preserve 31 “saints, beatified, and servants of God” between 1975 and 2008. The project, which tragically proved fatal to many of those who worked on it, is a bridge between heaven and earth. “The bodies and body parts of these holy individuals,” says one embalmer, “kept like a work of art.”..
When Pope Pius XII died in 1958, the Vatican used a wrapping technique similar to what was believed to have been applied to Jesus. It failed miserably. Only days after his death, his nose fell off, and a Swiss Guard fainted due to the stench while he was guarding the body. Pope John XXIII followed the reign of Pope Pius XII. After his death, John was treated with a simple formalin solution and placed in an airtight, layered coffin. It worked remarkably well — though the Church wouldn’t find that out until decades later...
In 1975, Monsignor Gianfranco Nolli, director of the Vatican’s Egyptian Museum, had an inspiration. After examining the excellent state of 4,000-year-old Egyptian mummies, he believed the Church could advance its treatments of popes and saints for the same effect.
St. Teresa, who died in 1582, is an example of how obsessed earlier Catholics were with relics of the flesh. After her death, a priest cut off her left hand, from which he took a finger, wearing it around his neck for the rest of his life. Followers later removed her heart, right arm, right foot and a piece of jaw to display as relics in various sites...
31 bodies and body parts of saints and other holy people the mummification team from the Vatican worked on from 1975 to 2008..
The team’s most important task was Pope John XXIII...More than 25,000 people visit St. Peter’s Basilica every day, and many faithful still believe the incorrupt state of his body is a miracle.
Shockingly, there is only one survivor from the original team, the others having died of various tumors and cancers, likely side effects of the toxic chemicals expended during their work. Nobody is currently willing to assume their task due to the peril. - http://nypost.com/2014/03/22/making-of-a-saint-the-vaticans-quest-to-preserve-its-leaders/
The Order of the Good Death The (Not Really So Very) Incorrupt Corpses
[some] incorruptibles get treatments (like a corpse spa) and it’s no big secret.
According to Church doctrine, incorruptibility alone can’t be counted as a miracle anymore. More sophisticated scientific explanations as well as mistakes found in hundreds of years of preservation records have forced the Church to reconsider which saints deserved the title in the first place....
small differences in temperature, moisture, and construction techniques lead to some tombs producing naturally preserved bodies while others in the same church didn’t....
Once the incorrupt bodies were removed from these climates or if the climates changed, they deteriorated. This may have been what happened to St. Francesca Romana who was deemed incorrupt four months after her death in 1440 only to be found fully skeletonized in 1698 (though you still hear people refer to her as incorrupt)...
The use of wax as an artist’s medium invites even more confusion and misunderstanding when it comes to the incorrupt. There are thousands of wax effigies in Catholic churches. Some have relics inside them, others don’t, and maybe a few hundred are fully incorrupt corpses wearing wax masks.
At the turn of the 21st century, the Vatican commissioned a team of pathologists and chemists to study saints’ bodies. The team encountered many cases in which saints had clearly been mummified, often by their followers, in an attempt to preserve (or help preserve) their bodies. One of these cases was Saint Margaret of Cortona. - https://psmag.com/environment/incorruptible-bodies-saints-religion-catholicism-pope-john-paul-94892<
The Incorruptibles The bodies of many medieval Catholic saints and martyrs have resisted decay for centuries— just the sort of mystery that begs for scientific inquiry By Heather Pringle -
Those who preserved Saint Margaret had done so remarkably thoroughly, excising her internal organs and drenching her skin in fragrant lotions. Their handiwork reminded Fulcheri of the techniques employed by ancient Egyptian embalmers. Mulling this over, the pathologist wondered whether the resemblances were merely coincidental or whether at some point in the distant past,...
In 1697, for example, an Italian surgeon left a list of 27 powdered herbs and drugs that he had employed to preserve the body of Saint Gregorio Barbarigo. ...
The church records offered no explanation for such drastic actions, so Fulcheri began hunting elsewhere for clues, searching to see if he could find other similarly mummified saints in Italy. His research has turned up five other similar cases— Saint Clare of Montefalco, Blessed Margaret of Metola, Saint Catherine of Siena, Saint Bernardine of Siena, and Saint Rita of Cascia...
By the time holy figures were exhumed during beatification or canonization trials, the tombs' microclimate had sometimes desiccated their flesh, turning it to the texture of old leather. And if there was any confusion among officials about which body was the correct one, they picked the best preserved, for incorruptibility was taken as a sign of holiness. Is it true that the bodies of some saints are incorruptible? /p>
I read _Scent_ by Annick LeGuerer some years ago and it had a section on the scent of sanctity. It discussed the presence of acetone in the blood as responsible for that agreeable floral odor described as "sanctified." Acetone in the blood is a result of diabetes. The diabetic body makes it in abnormal mass quantities. (This finding is what led to the Church saying that the incorruptible body could no longer be the only test of sanctity.) The book said St. Teresa of Avila was a confirmed diabetic (but I haven't read that info anywhere else). The book also went into how the mind (in ecstatic state) could change the blood alcohol composition so that was another way for saints to have the scent of sanctity. It was really interesting to think that our incorruptibles/saints were diabetics. - Miss P
Vatican’s secret, and deadly, project to mummify saints
Paramahansa Yogananda As reported in Time Magazine on 4 August 1952, Harry T. Rowe, Los Angeles Mortuary Director of the Forest Lawn Memorial Park in Glendale, California, where Yogananda's body was received, embalmed and interred,[46] wrote in a notarized letter[7] The absence of any visual signs of decay in the dead body of Paramahansa Yogananda offers the most extraordinary case in our experience... No physical disintegration was visible in his body even twenty days after death... No indication of mold was visible on his skin, and no visible drying up took place in the bodily tissues. This state of perfect preservation of a body is, so far as we know from mortuary annals, an unparalleled one... No odor of decay emanated from his body at any time...[47][48]
Which accounts are not alone: [Buddhist] Monk's Body Miraculously Intact 80 Years After Death