This thread might put you on the right track. I have not researched the matter, so I cannot comment as to it’s validity, but apparently Pope Galasius rejected an apocryphal “gospel” called “Transitus Beatae Mariae". Apparently, it is all about the assumption of Mary, however I do not know why the Pope rejected it- there might have been many reasons he rejected it. It will take more research, but I’m guessing this is what White is talking about.
Also, I found a snippet on
this website about this “gospel” saying generally that it supports all the details about the assumption that we know about today, however it says “The account of Pseudo-Melito, like the rest of the Transitus literature, is admittedly valueless as history, as an historical report of Mary’s death and corporeal assumption; under that aspect the historian is justified in dismissing it with a critical distaste.”