LG Ultrafine 5k -- right half is a slightly different color from left
I've been having this issue for quite some time. Usually, a reboot would make it go away, but recently it has become much more persistent. One workaround I found was to turn on Night Shift but set it all the way to the "Less Warm" side. 

It looks like an issue with macOS. I have filed a bug report to Apple.