Have you experimented with the HDMI ports the device is connected to on the TV? .. On modern TVs they are not all equal.
Yes sir. All 4 of them.
Even the LG G3 55" of a friend shows the same result with both Asus PN42 and RPi5. My friend owns a Homatics 4K Plus box running CoreELEC without the mentioned issues.
So I think I will use CoreELEC on Odroid N2+ for now. Fortunately I can keep using the Odroid of my friend as long as I need it.
I will try the nightlies now and then to see if this 'feature' has been fixed
Thanks for the suggestion chewitt!