Try this
While in flight, request parking at your destination once the option becomes available (airport [PLANNED]). Select parking spot, click the GSX tab to close the menu, (not the X, click the actual GSX tab on the dropdown menu. But this issue happens regardless most of the time, just more prominent when you close the tab from the drop down menu). I do this because the sizing cursor gets in the way. Land, taxi in, opened GSX again, select deboarding. For me, almost every single time all ground service vehicles disappear regardless of what scenery I am using. iniBuilds LAX, iBlueYonder PDX, BMW SEA to name a few, freeware sceneries, it doesn't matter.
As for the menu popping up, there has to be a reason. I know MSFS has an 'End Flight' option after engines shut down, but that is disabled. Maybe it's related to that? Because that would open up a menu as well.