Infusing armor stability does not work?
It’s instant cast, but it’s not a stun-break. You need to use it before you get stunned.
I tried it against some PvE mobs and it worked.
My guess is that somebody managed to break through the stability with enough CC skills or they boon stripped, but then after you were stunned, Infusing Terror pulsed and you gained stability. That can easily happen with any pulse stability skill.