Night Light only affects one half of tiled 5K displays (Dell UP2715K, Nvidia 390)
This is what my screen looks like with Night Light:
This photo is from an older pop!_OS build because it's the only live USB drive I have with the Nvidia blob drivers, but the situation is the same on other distros as well. I was hoping that mutter!71 (merged) would fix it, but I'm on mutter 3.28.1-1+elementary2~ubuntu5.0.1 now and the issue persists.