There is another thread related to this with some more details: Backpack graphics issue
Quick gist:
If your character picked up a mystery item from the Postmaster mail, that item is actually in the bag without an icon or tooltip. It’s bugging out the UI for that bag.
Current workaround is to find that item and move it to a slot in the main bank.
You can find the item by trying to “move” an item from every slot in the affected bag. For one of the slots you would actually pick up the item (tooltip will be a question mark). You can also sort the bag to help find it as the tooltips of the other items in the bag would appear on hover. If there is a slot in the middle of other items that appears empty, it’s likely the bugged item.