When minimizing Minecraft and re-entering (with incomplete restarting into the game), all banners become white in the inventory. To make flags common, just re-enter the world.
The error is minor, but sometimes such errors affect more serious ones.
Steps to reproduce:
- Enter the world in full screen (f11/or in settings).
- Minimize Minecraft.
- Return to Minecraft.
All banners in inventory will turn white.
The banners do not change.