Do I misunderstand the assumption? I thought it was bodily ascension prior to death.
Here's the key quote from Pope Pius XII's proclamation of the dogma: we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.
It doesn't say anything about whether or not she died or not. It is interesting to note, however, that Pius XII quoted from Pope Adrian I earlier in the document: "To cite an illustrious example, this is set forth in that sacramentary which Adrian I, our predecessor of immortal memory, sent to the Emperor Charlemagne. These words are found in this volume: 'Venerable to us, O Lord, is the festivity of this day on which the holy Mother of God suffered temporal death, but still could not be kept down by the bonds of death, who has begotten your Son our Lord incarnate from herself.'"
A later quote: The holy Fathers and the great Doctors, in the homilies and sermons they gave the people on this feast day, did not draw their teaching from the feast itself as from a primary source, but rather they spoke of this doctrine as something already known and accepted by Christ's faithful. They presented it more clearly. They offered more profound explanations of its meaning and nature, bringing out into sharper light the fact that this feast shows, not only that the dead body of the Blessed Virgin Mary remained incorrupt, but that she gained a triumph out of death, her heavenly glorification after the example of her only begotten Son, Jesus Christ-truths that the liturgical books had frequently touched upon concisely and briefly.
As Mary was dying, and she lived a good number of years after the death of her son, all the apostles were transported to her bedside.
This isn’t in Scripture but is recorded by the Early Church Fathers.
Look up the Dormition of Mary — I can give you links if you want them. LOL!