16" MacBook Pro Doesn't Wake Up Screen on LG 5K External Monitor
The issue has to do with the clock sync.  Basically, the Mac "knows" that the monitor is connected (I've verified that EDID data was being received), but since the Mac and the display "won't get on the same page" so to speak, the monitor shows blank.

I'd bet if you were to test this with a Bootcamp install of Windows on the same machine, you'd have no issues.  The Windows drivers are just more forgiving than the macOS kexts.

Many times you can solve the issue through hardware by using an active adapter if you are converting the signal (i.e. DisplayPort → HDMI).  However, if you're going DisplayPort to DisplayPort, there's nothing the active adapter is going to do to fix it.


External monitors show blank screen until restart
MacBook Pro detects syncmaster (monitor) but doesn&#39;t display
USB-C to Displayport cable or adapter


You may also want to try powering cycling the monitor (if the power button doesn't work, try unplugging from the mains) while the Mac is awake.  This solved it for a number of folks.