Can I just get my money back? This is not fun! I do this to relax, not to get frustrated and have to come in here and report beta software bugs.
In your other post about the supposed lack of SODE jetway functionality in FSDT airports, you confused what is in fact a shortcoming of some of our OLDER scenery (which WILL be updated, in the next weeks), with a GSX "bug".
Now, in this post, you are confusing the special case of the Majestic Q400, which uses a non-standard flight/engine model, so it doesn't work like any other airplane out there, in a way that when you switch off the engines, the *standard* engine variables (which work with ALL other airplanes out there, EXCEPT the Q400) will not be updated by the airplane code, making it impossible for GSX to know if those non-standard engines are turned on or off.
This is clearly NOT a "GSX bug", but rather something unique in that airplane.
But, as all other users reported and confirmed to work, if you use the CTRL+Shift+F1 key to shut down fuel valves, this will result the "standard" engine simulation which is still running in the simulator to turn off its own engine, so GSX can read the correct value.
Everybody using GSX with the Q400 has been using this for years, and it's confirmed to work.