I see this Bug yesterday to:
In my situation it happens after I disable the tool tips via macro and enable them again over macro. i used to do this because I use standard blizzard frames for healing in dungeons.
Thanks Scoot for all your work. I was a a little time away from testing nUI6 and it was wonderfull what you make in that time. Especially my bug with the minimap is gone away in every resolution.
So thanks a lot for all your work.