I use a 2018 Mac Mini with a Dummy HDMI plug and 2 monitors hooked to eGPU via DisplayPort.
Because of known behavior with waking the Dummy HDMI is needed on Mini itself. Also I use Hammerspoon to watch for wake for sleep and to cycle through SwitchResX profiles. 4K Monitors Dell U2718Q with HDMI, DP, mini-DP ports both manually set to active port (auto gets confused when switching source computer)
I use 3 profiles:
- All monitors enabled (obviously the HDMI is not viewable)
- Dual Monitor (just the DisplayPort/eGPU) with Dummy HDMI Disabled
- Single Monitor (Just one DP monitor and the other two disabled)
The reason for this setup is that I have a work computer attached to one monitor and so during work hours often have the Mac in Single Monitor mode.
The problem may not be Multi-Monitor issue but SwitchResX, but figure post just in case.
Also if I have Dual Monitor and the wallpaper displaying across both correctly, then switch one monitor to say HDMI , Multi Monitor works and just displays the wallpaper on the single monitor at the correct scaling .
But if Hammerspoon runs and uses SwitchResX it gets confused and either I end up with both images sharing the single screen, or if already in single monitor mode the wallpaper is scaled incorrectly.
To resolve I manually set both monitors to active and set Dual Monitor in SwitchResX, then I open multi-monitor and reload mmw file and apply. Sometimes takes a couple of tries.
Lastly wallpaper cache clearing has no affect (this is why I wonder if it is a SwitchResX issue - I am reaching out to them also)
My last thought would be to use hammerspoon on the wake to do something with multi-monitor, but not sure from applescript what commands I could issue.