That assumption nonsense is from 4th or 5th cen. apocryphal writings. Not quite “always”.
Yes, Apocryphal and condemned writings declared heretical by the Pope.
But these same heretical writings are quoted in preference to scripture by todays RC's to "prove doctrines" unheard of in the early church for hundreds of years.
If Mary’s body was not assumed to heaven, where are her relics? The Church, especially the Early Church, treasured the relics of her saints. Where are the relics of her most admired saint?