Because this caught my eye, I did a quick 200 Attk NON-Karmic test with a PC with a very high AC 26, just to see if this would bring out any subtle "Non-Karmic" boosts for enemies.
This was against Erna & Yerle in the Temple, mixing it up with ranged and melee attacks. They couldn't hit AC 26, even on a Nat 20, so it went pretty quick.
Just at a cursory glance, I didn't see anything unexpected.
200 Rolls
8 - Nat 20s
8 - Nat 1s
Assuming they
did hit on a Nat 20 (otherwise your game is bugged, and thus the data is possibly unreliable) that's good to know, thanks!
Combining your 200 data points with the 150 high-AC data points from OP, it seems like there isn't an excess of Nat 20s against high-AC characters using non-Karmic dice. 350 data points still isn't
great for a d20, but it's probably good enough.
Edit: just saw your post. Hmmm, you might want to report that as a Bug?? A Nat 20 should be a critical hit, regardless of AC.