When I wear the bonus item (Katze hat +1 def and +3 agi) and five statup points, I lose 3 statup points, I gain the 3 statup points back once I unequip the Katze hat.
For example I have 47 agi, I equip the bonus item with +3 agi and now I have 50 agility.
Since I have 50 agi now, I add 1 statup and end up with 48, so I lost 3 statups and gained +1 to agi that I just added which now makes 48.
Now I have 48 agi, I remove the bonus agi item -3 to agi and I gain +3.
Now i have 51 Agility the way it should be.
Test it your self, wear the bonus item called Katze hat then add like 1 statup, and watch the statup go down and not up.