Hello
I have just bought my new a6500 and since I m shooting video in interior and low light I discovered this problem. When camera is at 4K 25fps(or 30fps) in extreme low light there is "ghosting" problem. The same problem exist if you using a spot light to light only a part of your portrait while the other elements of the scene stays dark..
It seems like a sensor or most propably codec problem. Please check the video below. 4K, 1/50, sony 18-105 f4, iso 1250~2000 (no picture profile) :
https://www.youtube.com/watch?v=3zlrlpNQcIE
It seems we need a new firmware.