I only recently realized that the Defense stat seems to refer to Armor Class (inconsistent terminology is a no-no), because when I equip armor the armor class is added to my Defense stat.

But... if my armor has a magic bonus that gives Armor Class it doesn't seem to be added to my Defense stat...

So now I'm worried that Armor Class magic bonuses might not do anything...

Can anyone confirm?

I ve the same problem... Just now I buy some items that add attributes points, but when I add it to my equp, my total defence decrease... I explaine better, I have ad armor, 32 def + 18 armor class + 7 speed, when i use it my defence increase to 66 point, then i have a belt, +10 to attributes, when i use it, my defence increase to 10 points, but when i put in my helm, +6 to attributes, my defence decrease from 112 to 103... and i dont know why... In the past I had the same your problem, an armor that didnt sum the additional defence... and also i was thinking that maybe was, like you, Invisible armor class bonus, but in the end, i think is a bug and some item work, and some not....