The current set up I use to play WoW (and other games) is my laptop monitor on the side used for browser windows/spotify etc., and then a bigger monitor in front of me, which I will play WoW on. Since the pre-patch this morning, I noticed WoW had defaulted to my laptop monitor (seeing from the graphics options, it’s labeled as “Monitor 2”). I tried changing that to “Primary”, as well as “Monitor 3” (which I have no idea what it is, because as I mentioned I only have my laptop monitor and my main, external monitor). Whichever option I choose results in WoW going in the background for a few seconds and then defaulting back to the laptop monitor and “Monitor 2” in the settings.
I have the same issue, 3 options: Primary screen, monitor 2 and monitor 3. Only monitor 2 works which is my smaller second screen that i never use for gaming and i cant switch to a different screen. Also exactly as how you describe it