This bug started in 4.12 and has not been fixed in 4.13. Please vote here to get them to fix it sooner.
What is happening is when you loot a container and it hides automatically when fully looted, the bug is preventing the end Hover logic from happening on the widget (where as from 4.8-4.11 this worked fine).
Same as if you had your cursor over the inventory and then you hit I to hide the inventory, the engine bug won’t call the end hover states anymore.
The temporary solution is to bypass the IsMouseOverUI check in the InventoryPlayerController.
This logic has nothing to do with the inventory system and simply made it so when the cursor was over the UI it would block the player from being able to rotate his camera.
Hopefully this is fixed in 4.14.