I have seen that EXACT issue before, I am not the one that is experiencing this issue but the owner (he is also a racer) of a local hobbyshop near our track.
He as Several armature that are worn Exactly as described, he is also the only person that I know of that this has happend to. I have been racing RC for 10+ years and that as not yet happened to any of my motors. strangest thing is : he is also running an FK05. and that only happends on Stock motors of is (obviously).
I got a bag full of Fubar armatures from him not to long ago, ill take a picture if I can and post it.
Matt