I've recently encountered an issue trying to switch out of ACR to another application/desktop using the standard <alt><tab> key stroke combination.
When I try this, Icons for any open application (+ desktop) show correctly in the usual box. However, when I select an app and release the keys it jumps back to ACR.
When I get dumped back into ACR, the Open Image, OK and Cancel buttons are missing. A single click on the window brings back focus, and the buttons display again.
The alt - tab combination works fine from PS, LR and all other applications.
Aero switching <start><tab> fails the same way as <alt><tab>.
I've recently updated to the latest CC versions of Bridge and PS.
Windows 7, 64bit.
There is another ACR forum thread on this: ACR 8.7 always in the foregound. Problem reported there "only happens in full screen mode. Regular mode works like usual." Do you experience the problem when ACR is not in full screen mode?
You're correct that it works properly in non full screen mode...I can live with that for now!