Same issue with a 2019 MBP, USB-C to USB-C 4k monitor. The issue can be temporarily fixed by mirroring the display in settings and then turning mirroring back off. This works until I switch the display or the laptop sleeps...sigh.

Another confirmation that this fix does work! Running 2019 MBP hooked up to an LG 27UD88-W via USB-C. System Preferences -> Display -> HDMI Monitor -> Color -> Calibrate... -> Click through all the dialogs, Done!