After Disconnecting AirPlay Mirroring, my Ext. Display Resolution Does Not Change!

Im on Latest MBP 15, using Apple TV3 and DELL 27' Monitor as a main display.

When Im trying to play video via AirPlay + AppleTV as a Mirroring or Extended Display, it does change main resolution of my DELL ext. Display from 2K to 1080p... And after that I can return my native DELL resolution ONLY after Rebooting (even several times).

DELL is connected to MBP with mini D-port to D-port cable, ClickTronic (good and expensive one).

Is there any ideas how to fix that?

Thanks.