Sorry man, that sucks to hear. Hopefully you can get your hands on another one that stays healthy during acclimation.
I've never had an issue with bucket acclimating, what was the reason stated when you were told it was a bad idea specifically for ATF? Just curious as to why that would be the case.
I will say I've heard people debate whether or not to acclimate a fish outside of the bag or to strictly match temps while floating the bag and then letting the fish in the tank after shipping. I'm not sure whether it's because of ammonia buildup during shipping being exposed to greater levels of oxygen or if it's somehow related to the additives used to ship the fish. I'm honestly not even sure if it is an actual issue, as I've only had one fish shipped directly to me before.