The GMB logo has been specifically mentioned previously as being a problem – here’s one thread which includes an example photo and some explanations of why this occurs.
Basically, the coloured pixels displaying the bright GMB logo have started to dim over time, which causes a shadow to be visible when watching other content. It’s essentially uneven pixel wear, which many refer to as ‘screen burn’. It’s likely that the brighter your luminance and/or brightness settings, the quicker the pixels displaying a static image will degrade.
Because this is wear, it’s widely accepted that it’s cumulative, so watching GMB for 30 mins a day for a year could yield the same results as watching it for 130 hours straight !