I ran into this one with my newest laptop where minimizing or windowing a full screen RDP session will not full screen with the buttons at the top again
After a bit of searching I found that the keyboard shortcuts will work correctly to re full screen
CTRL+ALT+BREAK is the official one CTRL + ALT + PAUSE works for me on my laptop as well (so I dont have to press the function key as on my laptop the Break is a function of the Pause key)