When equipping any piece of armor from the hotbar using right-click with a shield in the offhand, the shield will become stuck in the resting position. To fix this, the shield must be removed from the offhand. This seems to be visual-only and doesn't appear in third-person. This does not happen in 1.15.2.

To replicate:

1. Open any world
2. Place any shield in the offhand
3. Place any piece of armor that can currently be equipped in the active hotbar slot
4. Using right-click, equip the piece of armor
5. The shield in the offhand can no longer be raised to the "up" position