I have a Mac Mini (5,1 mid 2011) running High Sierra with two monitors connected, one HD and one ultra-wide. It's generally been fine, but sometimes (and currently every time) when I start it, one of the displays is corrupted.
In this case, the corrupted display is using the HDMI port (the other one is using the mini DP port.
In the past, I've also had the same issue with my iMac (when returning from sleep mode too), so it's perhaps not a hardware issue. The cables have been replaced.