Armor check penalty to tumble AC bonus not working correctly

[Anthee]

Upon reaching level 7, I took a fifth rank in tumble for my character. When wearing armor and/or a shield, I would start noticing an AC penalty. However, it was inconsistently applied depending on in which order I would put on my equipment.

The gist of the matter is that if I equipped my shield (a +1 large shield) before the armor (Watcher armor), total AC would be 1 point higher than if I equipped armor first (after both shield and armor were on). In either case, I'd see the reduced AC icon in the upper right hand corner of my screen. What's more, the AC was reduced by more than the 1 point I'd have received from tumble if I equipped my shield after the armor: with just the Watcher armor on (and +1 natural armor AC from an amulet, +1 from dex), I'd have total AC of 17. When afterwards I equipped the +1 large shield, the AC would go up to 18 only. If I equipped the shield first instead, final AC would be 19. None of these AC modifiers are overlapping so this would be very strange even without the final AC being different depending on the order of equipping items.

I'm quite sure that this is due to the script modifying tumble AC because all of this weirdness stopped happening after I lost a level due to dying and no longer received any AC bonus from tumble.

An additional weirdness: with all equipment on (armor equipped first --> total AC 18, as described above), if I unequipped my shield, my AC would first drop correctly by 3 points to 15 (the equivalent of the bonus of a +1 large shield -- despite AC having only increased by +1 from 17 to 18 when equipping the shield), but 1 second afterwards, some script would kick in and raise my AC to 17. If it's the tumble script, that'd be weird because the AC bonus I should have received was in any case +1 at most.

I can demonstrate any of this to a DM IG if that helps to understand.