I had an "Armor 3" and a "Armor 2 Keep Not Behind". I was on fire for 1, and the Armor 3 stepped up and took it. Does ongoing damage count as coming from behind? The text says "Doesn't protect attacks from behind", but if that is true it should say "Only protects vs attacks from in front" so you know it doesn't work on ongoing. I'd much rather have Keep armor just always go first of course.
The "behind" thing only applies to the initial attack. See here. "I set you on fire from behind, eventually your whole body catches" -- SurgeonFish. It's probably card age: either armor checks go through your oldest draw first, or your newest draw first (I believe Jon specified at some point). Likely, you were just cursed by the order this time, and might be more fortunate the next.