As recommended here I changed the resolution to 1080p in the GUI. As most of the movies I watch are 1080/23.976fps I changed the refresh rate in Kodi settings to 23.98. This then avoids the TV switching modes when swapping between the GUI/movie (giving a black screen for around 0.5 second).
Now when I change the refresh rate from 60hz the screen becomes noticeably darker. At 720p all the different refresh rates look good, just most of the 1080p are dull apart from 60hz.
I have a Fire TV 4K stick connected to the same TV (LG B7 OLED) and it doesn’t go dull with the different 1080p modes.
Anyone else noticed this?