It is possible that, because of too many addons using Simconnect at the same time, the increased traffic on the Simconnect communication channel might possibly result in some commands sent by GSX ( the command to remove the static jetway in this case ) not being received, since other add-ons are spamming the channel with too many commands in a short time.
You can try to verify if this is case by disabling all your add-ons from the Options->Add-ons menu ( which will disable them temporarily without the need to uninstall them ) except FSDT Addon Manager, and try the same airport with just GSX enabled.