Items with transparent models, such as stained glass variants, render differently when a glowing entity is visible in frame. This also happens on items rendered on entities, such as armor stands and item frames.

To Reproduce:
1. Drop white stained glass and see that you can see the inside of the item model.
2. Summon a glowing entity

/summon armor_stand ~ ~ ~ {Glowing:1b} 

3. Notice that the item model may now renders flat, and the inside is no longer visible
4. When you go near the item and it is picked up, for a brief moment, it may appear to render correctly again.

According to Bartosz Bok, this issue is somewhat unpredictable as a result of state leak, which depends on order of entity rendering.