I should add that the waking problem only arises after the Mac Mini (or monitor?) has been asleep for an extended period of time, such as overnight.
macOS has a "deep sleep" mode that is likely involved here (across multiple macOS versions and system configurations). This is documented and controllable in Disk Sensei (Tools > Optimize > Sleep Image):I routinely log out at the end of the day, and occasionally, I’d say about 10 to 15% of the time, the monitors do not come to life the next morning. Well, they do in a way, because they power up and you can hear the beeps but remain dark. Normally, after several tries, I would have to force the Mini to shut down and reboot, at which time I do get a normal startup. ... I’m wondering if the problem lies with the built-in Intel 630 graphics...
So, one issue is that a Mac waking from deep sleep may be loading large amounts of data from storage back into RAM (as big as your RAM configuration), as well as storing that data onto storage when going back into deep sleep. My sense from trying to sort out various problems is that there may be bugs (software/hardware) in this area (as well as delays).Cindori said:Sleep Image (Default: On)
During deep sleep (hibernation), your Mac stores the RAM data onto an image file on disk. By disabling this feature, you will prevent your Mac from being able to go into deep sleep mode, but at the same time free several gigabytes of disk space.
Of course, there may be other display bugs unrelated to deep sleep per se, and the theory of them being related to Intel integrated graphics is interesting, as those are the types of Mac systems where I've encountered them.
For what they're worth, a few Apple notes:
Apple Support said:
I don't know if Apple's tricky Power Nap feature is involved:Apple Support said:Get help with video issues on external displays connected to your Mac
Try these steps if you don't see an image on your display. These steps can also help if the image on your screen repeatedly turns on and off (flickers), if horizontal lines appear (snow), or if the image is distorted (torn or scrambled).
These problems are widespread, though:Apple Support said: