So I'm having an odd issue. I'm pretty sure this only started since macOS 10.12 but I can't say for sure.
I'm using a late 2012 Mac mini with two displays. The first display is a Dell U2717D connected via DisplayPort. Resolution is 2560x1440 @ 60hz. I've never had any problems at all with this monitor connected to the mini. The second display is a Dell U2417H, connected via HDMI at 1080p resolution. I only recently started using it with my Mac mini again, but I use to use them both together on the mini without issue.
If I start up the mini with both monitors connected everything works perfect. The problem only arises if I power down the HDMI second display or plug it in after the mini has already been running.
What happens is the DisplayPort main display loses it's signal. Nothing I do can fix it other then restarting the computer. I've tried unplugging / cycling power on both displays in every combination. Once the main display loses it's signal the mini has to be restarted. The HDMI second display continues to work fine. The mini still recognizes the main display but gathers the Display settings pane to the second display as if it knows the main display isn't working.
Anyone have any ideas?