Thank you very much for checking this also. I tried it again and unfortunately I got similar results like before. I even set it to M without auto ISO. I tried with flicker detection on and off. None of them helped. I do not have red-eye reduction turned on.
I think I am just going to get 622c and try using that.
I'm sorry you're having trouble with it. It will be interesting to see if the radio wireless trigger solves the problem, or if it turns out to be the flash unit. I thought maybe it was the known issue a number of users have reported with Yongnuo 568s having inconsistent exposure used as bounce flash in E-TTL/evaluative, but since you mentioned direct flash that didn't seem to be it.