This is a known problem with Windows plathform.
A workaround is to NOT maximize the current window.
Another option that minimizes the issue (Unmaximized FileMaker DB's in Windows, with the extra set of scroll bars for the application window, looks really "clunky" IMO.), is to immediately resize the original window out to the maxium dimensions returned by get ( ScreenHeight) and Get ( ScreenWidth). Then, when you close the new window, you once agin maximize windows. You still get a window size change, but at least it's a small one this way.
That would all be easier to manage if there was a way to detect that windows are maximized in the first place, but so far, I haven't figured out a way to detect that state from a FileMaker script.
You can find example scripts that do this in the Known Bugslist Database.