I also have this problem, though with a bit of a different complication: I have recently made a stealth character and had planned for them to have one dagger and only that in their hand. However, it does this power attack only bug. If on the other hand I have a spell in the offhand it works perfectly, so for others perhaps try that? This makes the character unable to block though which is annoying, and only works with one handed combat (obviously) so it is only a band aid for a problem and a really bad one at that.