I recently started playing Pre Sequel and I ended up with about 30 keys before I started the game. I put in some shift codes and had some freebies from having BL/BL2 saves.
Every time I go to the key collection machine, all my keys are back. They add to my total every time. I’ve only ever played in co-op and only ever by joining someone else’s game. I’m not sure if that’s a factor, but I figured it’s a pretty huge exploit.