I have a 2017 iMac 21.5" 4K with Retina display. My Mac is connected to a second 4K monitor using a Thunderbolt 3 to DisplayPort cable.
If I connect the two of them there is no problem, everything works.
The problem arises when you shut down the Mac and turn it back on: the second monitor tells me it doesn't recive an input signal, even though the Mac detects it.
What I have to do then is to detach the Thunderbolt cable from the Mac and reconnect it and then the second monitor lits up.
The thing is that it doesn't always do this but the 80% of times. When it doesn't behave that way, and everything works, I noticed that on the second monitor the wallpaper is resetted to the default High Sierra one.
Any suggestions on why this is happening?