When using a texture in a custom model, this texture won't be found unless it is in one of the usual namespace:textures/ subfolders (block, entity, object, etc...). This behavior is not observed in 1.19.2 or an older version, and, as far as I know, is not documented anywhere.

As an example, a texture with a path minecraft:textures/block/aaa.png can be found and used in an item model but a texture minecraft:textures/aaa.png or minecraft:textures/custom/aaa.png will render as a missing texture.

In the attachment, I put two resource packs : one in version 1.19.2, which is working as intended, and the same resource pack in 1.19.3 (the only difference is the pack.mcmeta pack_format numbers, respectively 9 and 12). Those packs both change the models of the carrot, the potato, the baked potato and the poisonous potato, which respectively use the textures minecraft:textures/block/test_carrot, minecraft:textures/item/test_potato, minecraft:textures/a/test_baked_potato, and minecraft:textures/test_poisonous_potato.

While all textures will render correctly in 1.19.2, only the first two won't appear as missing textures in 1.19.3.

I also added a screenshot of the error messages in the Minecraft game output which appear while reloading the 1.19.3 pack  :

Missing textures in model minecraft:baked_potato#inventory:

minecraft:textures/atlas/blocks.png:minecraft:a/test_baked_potato

Missing textures in model minecraft:poisonous_potato#inventory: minecraft:textures/atlas/blocks.png:minecraft:test_poisonous_potato