The "Display mode" on "Droid settings > Display > Screen resolution" isn't preserved after reboot.
I always default to the max resolution (4K) and in my setup the image is not stable.
There are many display problems. For example, if you pick a low resolution, and pick the 'largest' font and 'largest' display size, the navigation bar on the bottom of the screen will glitch. This error has been present since first ROM build.
The problem occur to me, too.
Recently I change my old FullHD TV with new one 4K TV. With new TV, customized display settings isn't preserved, after reboot it autoswitch to max. available 4K60Hz.
I solved this by using Superceleron's ATV V1 userdebug firmware and disabled Dolby Vision. (in any case dolby vision don't work for me in any firmware).