...Somewhere along the lines of Transdiem transformative products, people with semi-limited desires manifesting god-like abilities, old cabals making themselves known, demi-humans making themselves known, etc... Reality had became very muddled, and whether it was an actual god, some sort of shadow government, or who knows what exactly... Reality reasserted itself. Crazy mythological, magical, fantastical parts of this new reality just kinda vanished or stopped working. Any special passives, abilities, bloodlines just didn't matter or stopped working. Cat girls either became Cats or girls, absurdly busty women became... well women that maybe had impressive busts for reality. Any transformations that would defy common sense or create bodies too far outside of the norm would get slapped into plausibility.
That isn't to say that every transformation was simply undone. At some point the core of a person intermingles with their transformation enough that they'll retain some part. Trans people who were magically/scientifically/mythologically... turned into the other sex would still retain the gender swap... it's just good luck swapping back. Someone who had became truly a child at heart as well as body would retain their youth... for as long as time permitted their youth to remain before puberty started wrecking havoc on their body. It would be a time of strife, hardship, but also a stabilizing time where anything could be trusted to stay the same for more than a few days or a week.