Main-hand bug?
I don’t know if this is a bug or intended behavior, but I would lean towards intended behavior. This allows you to share a weapon between the two sets of 1-handed weapons while switching only one. In other words if you want to have Axe/Axe in one weapon set and Axe/Warhorn in the other, you could do so with only two axes instead of three.
Because the game automatically fills your empty hand with what’s available on your swaps.
I.E. if you equip S/S on main set but only WH on second, when you swap to second the game will temporarily move one of your swords to your mainhand.