I think this is how Amlogic HW decoder outputs the picture. You can try changing brightness settings in Kodi, I think they should work.
Thank you for your answer. I've tried, but this doesn't solve the problem. It's like the image is going to the output with wrong color levels, maybe with PC levels instead of TV levels? I know that Kodi used to have a setting for this, but I can't find it in the LibreElec version