Same problem here:
I can choose any 2160p resolution (24,25,30 or 60hz) and it works perfect. Then i put the Q10 Pro into standby and when restarting the resolution stays at 2160p for about 3-4 seconds and while the Himedia-Logo is still shown the player switches to 720p50...when the player is powered up i can switch back to 2160 without problems...
Another fun fact: the 'hdmi auto adaption'-switch in the display-setup menu also chooses 720p50 automatically when activated. But when 2160p is selected the switch gets turned off. Maybe it is still activated somewhere in the boot-process...?