Support told me a while back by email this won’t be fixed in the 7.x cycle and maybe it will be fixed in Cubase 8.
I think this is a huge flaw when it comes to functionality of a full screen mixer. Unfortunately I doesn’t seem to bother most multiscreen PC users (not an issue on Mac), otherwise more people would complain.
FWIW,here is a related topic with a workaround for 7.5.