(please include information of what you did, that lead up to the bug, what you expected to happen there, and what actually happened, then move on to describe below:)
I noticed that the +2 AB for using a 2 handed weapon and the +1 AB from Defensive Stance (Earthkin Defender) don't stack. If you have 10 ab for example and equip a 2 handed weapon and turn on defensive stance you only have 12 ab instead of 13. It doesn't seem to stack regardless of the order you activate the bonuses as far as I can tell. I verified with Aniel that this is not an intended behavior and am submitting an official bug report.
Steps to reproduce the Bug:
Equip a 2 handed weapon and Turn on Defensive Stance while watching your ab.
Expected Result:
+2 AB from 2 handed and +1 AB from Defensive Stance (or 2 if you have Iron Stance) resulting in +3 (4) total from both bonuses.
Actual Result:
+2 AB from 2 handed seems to override the +1 from Defensive Stance resulting in only +2 regardless of whether defensive stance is on or off.
Screenshot(s)
2 Handed Bonus Only:
+25 ab

2 Handed Bonus and Rally Cry:
+25 ab, notice the +1 bludgeon from Rally Cry should also provide +1 ab making it +26 ab

2 Handed Bonus and Defensive Stance:
+25 ab, notice the increased saves from Defensive Stance should also provide +1 ab making it +26 ab

Defensive Stance Only:
+22 ab (with a +1 weapon instead of +3)

Defensive Stance and Rally Cry:
+22 ab, notice the +1 bludgeon from Rally Cry should also provide +1 ab making it +23 ab

Additional Information/Notes
Originally I thought this was just a bug with Defensive Stance but upon further testing it appears None of the ab benefits from anything are stacking. This likely includes things like Banner (EKD) or possibly Bard Songs providing ab. I've tested it to the extent that my current character can and none of the ab bonuses stack with eachother.