The ac effect seems to work fine. I have a character who's tumble even after Wearing chainmail is still at his base ranks. (he has 16 without +11 with) Problem occurs when a negative modifier is put in place.

Today I was cursed by a ghoul lord aura, it dropped my tumble in chainmail to 9 which should afford me +1 ac. Instead it striped me down to -2 (taking my full tumble ac bonus). Even after the effect wore off I had to unequip and requip my armour to fix the bug.

Hrm. Definitely a bug, but this should be easily remedied by switching on/off a suit of armor. I'll try to fix it though. What things can cause a negative Tumble modifier?

Doom, Prayer(negative side), Ghoul aura, Curse Song

Anything that decreases all skill modifiers

Is that list comprehensive? I'm going to use it as a checklist for which spells need to be fixed, so please make sure! :)

Doom, prayer, ghoul aura, curse song, bestow curse, enervation, level drain of any kind from monsters. I think that's everything.

I think that this is fixed as of v736 for all but a handful of exotic cases. If you encounter it again, make a new bug report.