Did a quick search but couldn't find this bug reported (although it's annoying enough I'd be surprised if lots of people didn't notice).

I'm having an issue where any "container" becomes "stuck" in my character's inventory once I cross the threshold of 15 weight items (15 is fine, 16 is stuck). By "stuck" I mean the pack cannot be moved into any other position in the current character's inventory or moved to another character's inventory. The pack can be opened, and items can be removed.

I've been testing and it doesn't seem like item type, specific item, or stack size influence the bug. Every combination I've tried, once I cross the 15 weight threshold, the container is stuck. Backpacks and pouches both have the same issue (only 2 containers I have). Overall, encumbrance doesn't matter either. I can move a 15 weight pack into a character's inventory that goes above their weight allowance, but I can't move a 16 weight pack into someone with plenty of weight to spare.