It's a longstanding bug. You need to start heavy attacking near the tail end of ability animations. Trying to initiate too soon can lead to nothing happening. If you do get in a loop, just light attack and that will break it.
Yeah, it's worse than it was before Elsweyr. Not it pretty much happens every time. It use to occur every once in a while. Even after the target is dead the heavy attack continues. I just bar swap to stop it. One day....maybe it will be fixed.....