This happens to other bodies an not just Christian Saints. So I think that this is a “miracle” is not likely.
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If I remember correctly, the body of Paramahansa Yogananda (AUTOBIOGRAPHY OF A YOGI) did not decay when he died. I don’t think he was a Christian saint.
Also, in the book THE BONES OF SAINT PETER says they only found bones when they opened the crypt of Peter during a renovation of the Vatican.