[LooseWilly]

I've run into this issue plenty with pretty much all of my characters.

From what I can see, weapon & shield or dual weapon will equip when hands are empty, or unequip.

Likewise, from (cross)bow and back again (beware trying this in combat, as it will generally unequip one or the other in one round and then require a second quickslot click in the next round to equip).

I often have a separate quick slot for each weapon, so I can swap while not unequipping a shield. I also generally have a combo quick slot handy for a favorite weapon and shield, and another for a favorite dual wield combo. If I need to swap weapons while using the shield in a fight (what, that goblin elite has piercing resist?... ok, I have a mace), I use the weapon slots. If I need to swap a weapon while dual wielding, I open the inventory screen and do it manually (and hope for the best).

All I can say is, I haven't (yet) had anything drop to the ground bugged and unable to be picked back up because of the inventory bug... so it could be worse.

Hope that helps, or at least gives food/inspiration for strategies.