OMG, next time it happens I will try if this is a way to get it to switch back (without unplugging and replugging /or rebooting or whatever I usually end up doing)

EDIT: it's not the Dock I meant but the Cmd+Tab app switching UI, don't know what it's called... but I guess maybe which display they appear on are somehow related

I can't emphasis enough how I hate that you can't have one dock per screen, aggravated by the fact that multiple windows of the same app (i.e browser) can't be degrouped.

TIL