When Gramps does not remember your preferences between sessions, there is something wrong with your .gramps/gramps52/gramps.ini file. The file might have become corrupted or be locked (protected).
If you upgraded and Gramps encounters a bad gramps.ini file, it might be reading (and trying to import) a fallback version of that file from an earlier installation. And if you delete the 5.2 version, it will just try to import that old version again.
The fullscreen setting is under the [interface] section of the gramps.ini file. The default for full screen should be commented out with a “false” example:
;;fullscreen=0
Yours is probably “true” and looks like:
fullscreen=1
Open your .gramps/gramps52/gramps.ini file in a text editor, delete that line and save the changed file. If you cannot save that file with the text editor, then Gramps won’t be able to either. That would be an OS-level problem and you’d need to troubleshoot that.
Note: Look at the keybindings wiki page for the Fullscreen mode switch.
You’re on a Linux but macOS has an OS-level Full Screen setting too.