Have you entered a callsign from the aircraft customization screen on the pre-flight set up from the world map? Maybe that's overriding it. After deleting the callsign all was working as it should!
This was in fact a problem we never considered: if you change your callsign in the airplane customization page, it will change the ATC ID reported by Simconnect for all your airplanes, until you change it again. Since the ATC ID is tied to the PMDG .INI settings for that airframe, if they don't match, it would cause GSX not being able to read the airplane configuration for that variant.
As soon this was reported, we posted a small hotfix (no version change), which reads the ATC ID from the airplane.cfg first, reverting to Simconnect only as a backup in case the atc_id might be missing or the aircraft.cfg couldn't be read for any reason, and this should fix the problem, regardless if you changed the callsign or not.
The fix is already online since a couple of day, just run the FSDT Live Update and make a "Check" on GSX Pro.
