Actually I only set"Broadcast RGB":2" when it worked. But it is still the same question.
Thank you for the explanation, but I don't completely understand yet.
I assume that first the TV sends its available modes and the RPi selects the best and does modeset(s). Later I do my modeset again.
Can't we disable the negotiaton completely and only use one manual modeset?
Of course the RPi would only work with this one display configuration afterwards.
I'm just curious.
Thank you Hias. Your latest build works as expected and the mode change is successfully applied during boot 
I do not know if the following is also part of this problem,
but is it possible to define only one fixed video mode and disable all mode changes?
It seems to me that modetest in autostart.sh only changes the already negotiated video mode. My TV reacts with some flickering during boot.
Hello all,
I have a TV that supports only 1360x768 60Hz and should use limited color range (16-235).
My device is LibreELEC 10.0.2 on RPi4.
After a fresh setup of LibreELEC the video mode defaults to 1360x768 60Hz full color range so that extreme black and white levels get cut off.
The setting limited color range setting in "settings -> system -> video" does not work as it only affects the GUI, not the video playback. Although when I disable DRM Prime Decoder the setting works correctly in video playback.
On LibreElec <10 I used to set the config.txt hdmi_pixel_encoding to 1, which obviously does not apply anymore.
As an alternative I added "modetest -M vc4 -w 32:Colorspace:12 -w 32:"max bpc":12 -w 32:"Broadcast RGB":2" to my autostart.sh.
But some quirks are remaining: When the RPi4 boots it takes longer as usual and the displaymode changes back and forth. Eventually it displays 1360x768 full range 
When I start a video the video mode changes again (my TV has to re-adjust) to 1360x768 limited range and the video plays correctly.
The limited range stays applied even if the video is closed. Yay!
My goal is to boot directly into limited range.
It would be great if someone could help me.
Kind regards.
