Sometimes video "blinks" while recording - one frame becomes a bit more exposed than the others once in a while. Like +1 or +0,5 stop. Not sure about the exact rate but I'd guess that this lighter frame appears something like twice a minute.
Seems like turning off Focus Peaking fixes the problem but didn't test this theory properly. Have been using ML for more than a year with Focus Peaking and never encountered such a problem before.
Attached the short clip where you can see the "blinking frame" closer tho the end of the clip.