I currently have a three monitor rig with a GTX 1080 and in the last 4 months I’ve had some odd things happen.
Sometimes when I start the system, the POST screen switches from the middle monitor to the right or left monitors (this doesn’t happen every time, it seems to occur weeks apart). This also means the OS also uses that display as the primary (the left or right monitors instead of the middle monitor). I have to shut the system off, and re-arrange the display cables so the primary is the center monitor again; then go into the display settings on my operating systems and reconfigure them so the middle is primary again.
This may be a driver based issue on Linux, but sometimes the right monitor will go black for a half second before recovering. I’ve tried to isolate this issue to see if it was the monitor by switching them around, but it seems to always be the one connected to the right side (which is off the first displayport connection). I had a GTX 1070 prior to this and never had this problem on my three monitor setup, using the same displayport cables.