Picture browser disappearing after using another programm

I use DXO PL 6.3.1 Elite with the main window on my external monitor (BenQ SW2700) and the picture browser on the iMac screen. Since PL4 I have the same problem: After using another programm on the external screen (e.g. one from the NIK collection or Helicon Focus) and then maximizing PL6’s main window the picture browser disappears on the iMac screen and I have to undock it again.
How can this be avoided?