Left (main) hand with bow spins when holding shield and you left-click (hit) and instantly right-click (aim bow) after.
It also happens with no arrows in inventory.
It does not happen when you have [Main Hand: Right].
It does not happen with other items in offhand.
It does not happen when the hitting animation has finished already before you start right clicking the bow.
I wasn't able to reproduce this behavior in 1.20.4. Can you please check if this is still a problem for you? Thanks!