This just happened to me, twice, in PvE leveling of all things. THIS NEEDS FIXING ASAP, DEVS. DO YOU HEAR ME???? This is total horse shizzle -- unstoppable is literally the only thing that makes this class playable, and it's broken.
I am having this same issue. This feat is not working properly with recovery bonuses from the ioun stone. I.E., there is no armor penetration granted from the ioun stone's recovery stats.