I use LR in Full Screen mode. When I press the F key to get Full Screen Preview, and then press the F key again, LR 5.2 reverts to regular Full Screen, as expected. However, if I exit Full Screen Preview by pressing Esc, G, E, or D, it reverts to a non-full-screen window, and I have to press shift + F two times to get back to full screen.
Before 5.2., it did not work like this. All methods to exit Full Screen Preview reverted to whatever screen mode was active when FSP was invoked.
What operating system? I just tried it on my Windows 7 computer, and I don't see that behavior. Doesn't mean that you are not seeing it, it just means that I cannot replicate it. You might want to reset your preferences and see if that fixes things.
Mac OSX. It happened on Mountain Lion and now on Mavericks. Deleting the LR preference file, and letting LR create a new one, did not solve the problem.