Firstly, thank you for adding the banner shields. Second, now that you have added banner shields there are a few problems.

This problem in particular is:

During a multiplayer area fight with friends on my world, pressing shift (or the assigned key) would stop pulling the shield up. I don't know if frequent deaths while sneaked or respawning within the chunk have anything to do with it - but they would frequently stop working during the fight. We found that eating would stop the issue temporarily.

Another thing we noticed is the player was still doing the eating animation at the times that the bug would occur. I haven't seen anyone post anything like this yet.