I didn't know the mirror stayed up when the camera bracketed HDR. That's the answer then. In DSLRs that I have used the mirror doesn't just stay locked I always had to lock the mirror then fire the shot. What I do now with AEB is set the camera +1 / ...
I have this exact same question and asked it on DPReview and they pointed me here. Your question i marked as solved but which uggestion solved it? The "silent shutter" doen't lock the mirrior so you can still get softness when your brackets are a few...
