This happens in my Android 9 box in different tv sets and with different video apps (iptv, YouTube..., other streaming apps) but not with Twitch video captures, which are ok.
My other Android 10 box video captures are ok, too
I have tried chanching screen resolutions and refresh rates to no avail
Please, help
Yes, that is normal, I think it has to do with the HDCP compliant where some apps may take black screenshots when video is playing.