When I leveled up to take 5 base ranks of tumble, my AC gets the normal -1 penalty (Staying the same as before I leveled instead of increasing) since my gear puts me at 3 tumble.
But when I equip/unequip anything, it seems to apply the penalty AGAIN and put me at -1 of what my AC should be without any tumble. Relogging fixes this until something is equipped.
It's an issue with nwn and how it needs a moment to process stuff when it's equipped.
If you do it at slower pace, it's fine. If it happens many times, you need to remove items that give you a lot of tumble penalty to clear or relog. Either way.
One of those things that eventually I will use more time on to twist into different shape but not a priority until there if free time.