Something is triggering the ALC (automatic level control) in the video ADC to raise the black level incorrectly.
First try reducing the pre-ADC gain in the Video in proc menu by a step or two and see if that helps.
If not, try reducing the ALC V filter setting in the Video in proc menu. This will make the ALC respond faster and possibly fix the varying black level, but might also cause flicker if reduced too much.
Note that adaptive backlight in the display can interact with this.