@harryb skip the following gramps.ini troubleshooting. It is often the cause with these symptoms. But it is a red herring for your problem
MS Windows users who upgraded Gramps (having used 5.1 and earlier) will continue to use %AppData% but fresh installs will use %LocalAppData%. It is a bit confusing. (There’s another thread that discusses a way to have Gramps report on where is stores various details. This ‘Gramps constant’ for the Gramps User Directory is USER_DATA. )
Look in the Gramps User Directory for gramps52/gramps.ini
Back with version 5.1.2, this bug was reported and fixed. See bug report 11831
@prculley did a patch for the a MS Windows port that recognized when the virtual screen size (supporting multiple screens or screen configuration changes) had changed. It would recognize when dialogs were saved in off-screen locations and ignore those preferences. It seems like you’ve found a way the break his fix.