Hmm well dynamic could be the culprit then indeed. And that won't really have anything to do with the 2nd thread being disabled, but perhaps because the worksize is constantly changing, it might not be offsetting the right amount and repeating work. That will need fixing...
I was noticing higher rejects with 2.2.x also, and upon searching, found this same problem with dynamic intensity. Switched to a static intensity, and the problem went away.