There are a couple of replicatable bugs relating to flanking damage; anti-flanking skills not working, Reaver armor seemingly "back to front", but the general problem with armor in Threatening and Perilous is not a bug, per se, it's that the armor values are "balanced" for routine.
It's because armor is a flat "off the top" reduction, so if you've got 50 armor and get hit for 100, your armor reduces that hit to 50 before any other reducers kick in.
Because the damage that the mobs deal increases so much in Threatening and Perilous, you can't make armor that's "effective" in those difficulties without completely trivialising the lower difficulties. In simple terms, if the same mob hits for 50 in routine, 100 in threatening and 200 in perilous (made up numbers) you can't make armor that "works" for all three difficulties.
As it is Templars and Legionaires can already very easily get armor values high enough that mobs in routine rarely do more than 1 damage.
I think the best solution is to design threatening and perilous armors for every char avaliable ONLY when you get X promotions on that char, because it's too late to change the whole mechanic of "armor" in this game.
I know, perilous would be too easy (and threatening) for the veterans, and here is the other suggestion (posted already in the thread for suggestions): infinite tiers (not just routine, threatening and perilous). No game that gives you the opportunity to improve your char infinitely should have limits for its difficulty.
With more tiers and new armors ("unlocked" when you get X promotions) we could have leaderboards/rankings with the best teams clearing, for exemple, tier "nightmare" (after perilous) in X minutes. Of course, instead of thinking a new name for every new tier, we can just add numbers to "nightmare".
Exemple:
Best Team PC cleared Nightmare 3 in 25 mins.
People with the best armors would not take damage in routine and threatening, but... anyway veterans with 3938475 promotions don't step there even now I guess...