I managed to try it finally, and I cannot replicate the problem: GSX works just fine with the MK studios LSZH, no issues whatsoever.
And no, I don't have "Using airport customization from GSX.airports.lszh_fsdt_msfs" in my log. Yes, that's the .PYE file you removed, but removing would cause an error, it's not required to be removed, because even if it's checked at start, if the FSDT version of the airport is not detected, it won't be used, so you shouldn't see the line in the log.
If you see it, it means you haven't removed the FSDT version, this might be either a custom GSX profile for LSZH in %APPDATA%\Virtuali\MSFS folder OR the actual package, either in Community of Official.
COULD BE you have a GSX profile supposedly for MK Studios airport, but it was made by somebody who used the FSDT GSX Profile as a starting point ? That *might* explains it, since I'm not using any profile.