I am not fully sure on how it works but as I understand it the mechanical shutter is used in single shots but not movie. I suspect it is not used in burst shots as well. Try some examples with the setting to off and see what happens.
We had the same problem with our P4Pro. It turned out to be a fault in the mechanical shutter and the camera now needs to be replaced. As with your case, the movies still worked excellently. To verify if this is your problem, try turning off mechanical shutter in the camera settings menu.