Custom item models such as shields, banners, conduits, air, and mob and player heads all create invisible item
Example:
/give @s minecraft:iron_ingot[minecraft:item_model=shield]
Custom item models such as shields, banners, conduits, air, and mob and player heads all create invisible item
Example:
/give @s minecraft:iron_ingot[minecraft:item_model=shield]
This is probably a Won't Fix situation (Air is probably WAI) but I can confirm this happens with all item models that use the builtin/entity parent in their item model.
If you use the item_model component on an item that doesn't have the builtin/entity on one that does have it, it will appear invisible.
e.g.
This happens because it is not given a texture to assign to the item. This also happens if you use a model like the following: