RPi picture is often too dark / bright, compared to TV channels. Use a different picture blending mode for HDMI input, which is part of your TV settings. Works fine for me.
Same issue here, on recent stable:
Connected TV to HDMI0 and Audio Extractor to HDMI1. On booting the picture delivered by RPi 4B is too bright.
When booting without Audio Extractor on HDMI1, the brightness is good.
When changing the resolution or the refresh rate in Kodi video options, the brightness becomes good again until next reboot.
Issue reproducible. Doesn't happen when plugging the Extractor between HDMI0 and TV. Looks like RPi doesnt like devices connected to HDMI1
Edit:
Maybe this has to be split into separate thread
Connected TV to HDMI0 and Audio Extractor to HDMI1. On booting the picture delivered by RPi 4B is too bright.
When booting without Audio Extractor on HDMI1, the brightness is good.
This is a known issue of LE10.0.1 and will be fixed in LE 10.0.2.
You can give this build https://www.horus.com/~hias/tmp/libr…422-480810e.tar a try, it includes all fixes and improvements we got so far.
so long,
Hias
Yes, this fixed the issue. Thanks!
Changelog available? Any ETA on LE 10.0.2?
Any ETA on LE 10.0.2?
Probably some time next year. 
Changelog available?
Here's my dev branch that I used for this build: https://github.com/HiassofT/LibreELEC.tv/commits/480810e - lots of kernel and ffmpeg fixes and deinterlace and HDR patches for kodi.
so long,
Hias
