We have 4 axolotls named and placed in the world(server) and 1 in a bucket that all reverted to the vanilla pink axolotl? Not sure if it was after a server restart

As I remember, MAVM use axolotls nbts to handle variants, do you have any other mod(s) that can conflicts with axolotls or mobs in general ?

Well, in fact, doesn't use the vanilla NBT but is replaced with an identifier system. For example, nbt Variant 0 becomes "minecraft:lucy" and so on. That's unfixable sadly.