That would be because the first three level 90 dungeons - Smileton, Dead Ends and Stigma Dreamscape - ONLY drop accessories. The only way to "fix" that loot table would be to introduce more and different loot to those dungeons, there simply isnt anything other than accessories that could drop there right now.

For the new dungeon, unless that is actually messed up, no accessories should appear in the last boss chest.