Today, I loaded up my save file to try and find a way around the leveling bug for wizards. As an experiment, I decided to unequip all items and then try leveling up. That is when I discovered a Purple Stone Knife in slot 4 that I could not remove.
I will concede that it is possible I picked up the knife while exploring, but I’ve been paying close attention to loot and inventory. I don’t normally pickup everything, I just started playing, and my resources are limited. However, I would definitely remember equipping such a knife and if i did, it would go in slot 1 or 2, not slot 4. With hundreds of hours in Pathfinder: Kingmaker, I almost never use slots 3 or 4 on my wizards.
Somehow, this knife ended up equipped on my character and now I cannot unequip it.