I didn't have time to do much testing but here's what I can tell you.
I do have FTX Vector installed and had to disable elevation correction so that the aircraft does not sit underground. The problem I reported persists regardless of the elevation correction being enabled or not.
I do have a terrain mesh installed (FreeMesh X Global), disabling the terrain mesh did not solve the problem.
GSX vehicles do sink into the ground as well. The simple stair model sometimes moves along with the ground and becomes locked at the highest elevation causing it to float in the air when the ground lowers. See attached screenshot. I never got the other stair model (attached to a truck) to do the same.
I am using the regular P3D spot view and do not have any camera addon installed. The issue occurs in spot view, locked spot and top-down view. The elevation of the aircraft as displayed by Ctrl+Z changes in spot view from 349ft to 353ft. In top-down view the issue occurs when zooming in or out, elevation fully zoomed in is 349ft, quickly going up to 375ft when zooming out, so the ground apparently moves by up to 25ft.
Movement of the ground causes the aircraft to bounce, severity depends on how fast the view is being changed. Quickly zooming in or out when using top-down view causes a crash when crash detection is enabled.
When the trial period expires the ground markings (gate center line, red/white border, taxilines) remain floating at the elevation the ground was at when it unloaded.