If your problem is fixed after running the update, it's because either you configured FSX to run in a "Compatibility" mode, like XP or Vista or Windows 7 under Windows 10, etc, which shouldn't be done, since FSX runs just fine without those settings.
It's possible another add-on that is not completely compatible with the latest OS has forced the sim to enable Compatibility mode.
And, it's also possible that, if FSX crashed for any reason during the previous session, Windows might have set some Compatibility flags in the registry ( which you won't see in the Compatibility page of the .exe properties ), to "help" you fix the crash.
Running the FSDT Live Update will reset to settings to their correct defaults so, if you can run GSX after the updater, it's because one of the above.