if u switch another item in the for example vest the can should be shown, but i agree that this is no fix of the problem.
Happens to if u open a can on floor. Normaly it should be opened in inventory but it is not shown, after mooving another item it is shown.

evert tried running with 1 weapon on the back?
tried running with a weapon on back and a weapon in hands?
i mean real life not any game.
in RL u will dropp the 2nd weapon because u cant fix both weapons and if u run one of them will knock u out.
So if u can carry 2 weapons on back put in a random knock out during running.