macOS seems to have a problem with USB-C external monitors. My resolution is 4K, scaled up slightly to accommodate bigger text.
When it awakens from sleep, the monitor resolution is "wrong". When my monitor wakes up, it is set to 4K, but the mouse moves as if it's in 1080p. In other words, it thinks the boundaries of the screen are 75% smaller than what exists, so I can't click on the "correct" thing, move windows, etc. I'm essentially forced to reboot.
I have both a Dell and HP z34 4K monitors.
The former used DP to USB-C, the latter uses just a USB-C cable.
This problem has occurred with 2 different monitors, cables, resetting NVRAM/SMC, and a fresh OS install (10.3 and 10.14.1).