I acquired the purple colored heavy armor called Irb’s Viscerian Steel Plate Armor of Mystery.
BUG SUMMARY: It seems to be setting my AC to 15 (not 17), and then only recognizing +evasion modifiers (but it doesn’t include the agility modifier).
It has a normal AC bonus of +11, and also grants +6 to AC (total of +17).
My AC without armor is 32, but when I add that one piece of equipment (no other changes), it sets my armor to 17 instead of 49.
Even stranger: it actually sets my AC at 15 when I remove all other gear. Then, if I add a piece of gear with +2 evasion, it changes my AC to 17. Those are the only two results I can get with that armor, regardless of what else I wear.
It sounds like you figured out that heavy armor can only give agility bonus in certain situations. Namely if you have the Heavy Armor Specialization and your strength is at least twice the weight of the armor.
As for why it gives 15 instead of 17, is it possible you have a penalty to agility when not wearing items? If you can screen shot your conditions and your stats page, along with the item stats that would help me to figure out how it all adds up.