Steps to reproduce:
Place a shield in the offhand;
Equip a bucket in the main hand;
Use the bucket to pick/place water/lava;
The shield is now unusable.
A quick fix is just to re-equip the shield. 😛
I Recorded myself reproducing this bug, but the file size was too big to upload it here, so I just uploaded it to [YouTube|https://youtu.be/ggqBFyGHrjE]
Duplicate of MC-126337, which is intended game behavior.