When you block with a shield in 1.20.3 RC1, its position is incorrect in 3rd person if it's in your offhand.
This seems inconsistent, and I am unable to reproduce it at will.

I have noticed that whenever I block something, the shield's position briefly glitches in 3rd person, but it does not seem to cause this issue consistently or at all.

Offhand:

Main Hand:

Offhand:

Main Hand:

Bonus-Offhand Clipping:

I was able to reproduce it again in 1.20.3, but I still don't know how to consistently cause it to happen.