That might be a clue, or it might be normal. I've never measured the PWM signal when the camera was in service. Give me a few minutes. I'm putting my camera back together, and I will try it on my drone. Not sure how to do it with the drone shell already assembled, will have to figure it out.
I think you have found a clue. The PWM voltage range stays the same when the camera is in service. About 1.55 v when full up, about 1.68v when full down. (uncalibrated MM, so not sure the precise voltage).