When I select any of my other characters, the 3rd weapon slot appears with whatever Kana has equipped in that slot. If I click on the slot, Kana changes to it (even if I had a different character selected).

 

Not a big deal as it doesn't actually help or hurt me in any way - but it is a strange display/UI bug, and I'm not sure what would happen if I had another Amuaua with a 3rd weapon slot in the party.

I encountered that as well. It only seems to affect the weapon slots in the normal UI, inventory is showing the correct number.

Yes. Though in the inventory, Kana's 3rd slot occasionally has the locked symbol on it (behind the weapon). Doesn't affect functionality, but is another visual bug related to that 3rd weapon slot so worth mentioning.