The enemy popped up during a side-swipe attack and was able to kill you while it was standing on your sword? That's pretty strange. Or was it a regular, vertical attack? That would be stranger still.
I don't recall seeing this behavior during testing. Is it possible the enemy that killed you "fell" from a higher level like on the death cube ramp? That has certainly happened to me and it's frightening.
Any additional info helps, thanks!
-Cheers