I just got this keyboard and upon using any media key (apart from the mute and volume knob) while on fullscreen (e.g. youtube video or game) it just changes the focus back to windows.
On youtube videos I just see the windows taskbar pop on the bottom of the screen when I press said media keys.
On games it's much more pronounced as it straight minimizes the game.
The keys perform their function no problem, but this change of window focus didn't happen with any of my previous keyboards and I'm at a loss as to why it happens with this one.