Following your description, it seems GSX couldn't detect the jetway status, so even if it was attached, it didn't report as such, and this could have been caused by a number of factors, ALL of them of course logged by GSX, that's why you should always try to provide a log when reporting a problem.
Lacking a log, I can only list all possible reasons I can think of:
- There's a scenery conflict, so GSX is getting wrong data about parking and jetways from the Navdata API. Most common cause is not having disabled GSX replacement files for that airport after installing it, but if you have, you might have another scenery in conflict.
- There's a problem with that scenery in particular, like a missing Jetway or an orphaned jetway, and this affects what we receive from GSX.
- The jetway might have "worked" visually, but might have not docked completely, leaving some gap. Normally, when a jetway docks properly, there will be almost zero gap between the jetway end point (which GSX receives from the Navdata API) and the airplane door.
Of course, even when the jetway docks properly, the gap is close to zero, we have a tolerance (50cm I think) to accept the jetway as docked even if it's not fully docked, but it's not as if we can increase it further, because it would be we might start to accept as docked jetways with a gap too wide that might start looking as "passengers in the air", which you might assume it's a "GSX bug", when in fact the problem was the jetway docking partially. This case is not only indicated as a message in GSX, but also logged.
Clearly, if GSX doesn't detect the jetway as docked (again, it's Simconnect that tells us that, assuming we haven't received bogus data about it), it thinks it must dock it, resulting in an undocking, if the jetway is in fact docked.