I've got two external monitors hooked up to my MacBook Pro Retina (about says it's circa '13). One monitor is being feed desktop backgrounds from one folder, and the other monitor is being feed desktop backgrounds from another folder.
If I go to a meeting, I unplug both external monitors, and then, when I come back and plugin the monitors again, one of the monitors (not both) sometimes goes back into the custom background folder that I've set for it. Other times, that same monitor will use a default background from Mac.
I don't remember where I saw this, but I remember reading that Mac doesn't have a way of keeping tracking of monitors, which is why this is happening. Am I out of luck?