I was in the shop selling my excess stuff. I was going through my loot one item class at a time, changing the filter to show the next class of items when I was done choosing what to sell from the previous one. I had one treasure item left in my inventory and sold it. As I then had no more treasures left, the filter automatically dropped the "Treasure" option from the list and changed the selection to read "Weapon" but this didn't actually change the used filter and the screen now showed none of the weapons I had in the inventory. Changing the filter to something else and then back to Weapon worked just fine. Didn't happen to take a screenshot but the bug should be easy enough to reproduce.
Yeah: like so. I'll take this opportunity to mention something else I've noticed: if you leave the category filter alone and change the sort order by value, level, etc., then all the weapons appear. It's easier than changing the category back and forth to see your stuff; and it's further evidence that the client believes it is showing the weapons category even though they don't appear.