The magenta/black default texture appears on models made when making custom model data in resource packs and trying to use existing vanilla textures located in the "entity" folder. Access to this folder seem to be blocked now
MC-259348
entity folder access blocked for resource packs
Resolved
Works As Intended
0
This is an intentional change, only files whitelisted by assets/minecraft/atlases/blocks.json can be accessed by block/item models.
You can just append the files you need to it.
Go read the changelog: https://www.minecraft.net/en-us/article/minecraft-java-edition-1-19-3