It does pull away as soon as request boarding
No, it doesn't. Surely not with the latest update that uses SU12 to know about the jetway status.
If you updated GSX to the latest version and it still happens to you, the only possible explanation is a problem with Simconnect, due to the sim reaching the maximum number of objects in a scene, usually caused by AI injection, see here for some explanation:
https://www.fsdreamteam.com/forum/index.php/topic,29576.msg192029.html#msg192029However, to confirm yours is this problem, it would be best to check your log, and see if you also got the error 99, which means "Simconnect internal error", which causes Jetways to not be detected correctly.