This has happened to me twice.
On each occasion it was immediately after the game had chosen to "move" its origin.
I suspect there is a race condition in the code so it:
- checks the vehicle position against the terrain with new origin, but old vehicle offset relative to the origin
- finds a mountain so, logs the message saying vehicle underground, lifts the vehicle in the air
- then it fixes the vehicle offset relative to the new origin
- finds no mountain, so the vehicle drops...
Hope that helps, I can log a proper bug report with the output logs if you need it.
Note:
This was with me running as a client attached to a dedicated server - in case it makes a difference to trying to reproduce it