Too-thin item in an item frame has a weird shading

If an item is too thin, and its placed in an item frame that is put on a wall, it displays sometimes dark and sometimes bright, varying from player's rotation (see the attached screenshots). If the item is a little thicker, then the phenomenon is less obvious. If the item is not thin at all (like an vanilla model), the issue may be not seen. If the item is put on the ground, then this phenomenon does not exist.
This issue can be produced in any rendering quality (fast, fancy), no matter if the player has night vision or not.