I have a spot on the lower left corner of my videos which, at first, I thought was dust or dirt on the lens. But I have cleaned the uv protoector thoroughly, then went on to clean the lens as well, removed the lens and cleaned the inner glass, blew air over the sensor, cleaned the sensor with the electronic function, and no change. The blemish doesn't seem to appear in stills--did a time-lapse today that didn't have it. And occasionally a video seems to not have it, but then it's back, no matter the settings (4k or 1080; small or large aperture). In the clip below I zoom, and again, little change. Any thoughts would be appreciated.
My best guess would be dirt on the sensor. Maybe something that's not going to budge with just blowing on it. I used to see that all the time with my Sony FS100. It seemed to constantly attract dust and dirt.
Get a magnifier and take a close look across the sensor for something stuck there.