Okay lets start from zero.
Custo model data is a old ,very old way to edit items models. and 24w45a break all this system to "revamp" it to a new and "fancy" system that i personally will  use but i dont find correctly to just send to trash all the actual resourcepack. the datapack changues whas a bit hard to take up but managable at the end. 
So more than a bug or i dont know if it is a bug or a lack of backwards compatibility i leave that as a "bug" so please read an reconsider or at least add a way to keep compatibility