Bug Summary:
If you specify a player_head for the item_model, the item becomes transparent.

Reasons for noticing this bug:
I tried to change the apple to the look of a custom head using the new component item_model, but what I actually got was an apple with a transparent texture.

Steps to reproduce:
Type the following command.
/give @p minecraft:apple[minecraft:item_model="minecraft:player_head"]
Then you will get a transparent apple.

※ Note
I'm Japan, so I can't speak English. This text is written using a translator.