I can only tell you again that this is due to the way how Motion Blur Plus blends the frames. Unfortunately I cannot change that. Taking shots with a different luminosity at the beginning is not necessarily a good idea, since it could interfere with deflickering, holy grail wizard etc. Im most of the cases you won't notice it (especially if you crossfade in post). If you see it, just shorten the sequence a bit when doing the video editing. I use MBP all the time and this never really bothered me.