I guess it’s also depends on the type of flight plan that you have over that mountainous area. The ATC just follows the flight plan and its altitude constraints (if there’s any). So if the default navdata is proper and correct since you’re on Xbox, a flight plan that has STAR should have altitude constraints for every waypoints. If you’re on PC, you have the option to install Navigraph, which they all have altitude constraints on all the waypoints within SIDs and STARs.
The way you create your flight plan could also affect ATC’s ability to assign your altitude. If you create an IFR flight plan without a STAR, that would make your flight plan to be in a direct route to the runway, and without any altitude constraints, the ATC will just randomly give you a gradual altitude descent and it doesn’t matter if there’s a mountain on your flight path.
That being said, I never had an issue with ATC assigning the wrong altitude when flying over mountains because:
- I’m on PC, so I use Navigraph navdata.
- All my flight plans are IFR with High-Altitude Airways, with a defined SIDs and STARs.
- Each waypoints in SIDs and STARs all have altitude and airspeed constraints.
- Once the full flight plan is loaded with all that information, ATC always tells me to descend to the exact same altitude and airspeed constraints that are in the flight plan. So over mountainous regions, the altitude constraints are always within safe distance over the terrain, so ATC never gives me the wrong one.
So for Xbox, it’s a bit tricky because I’m not sure how “complete” the default navaids is, and how you create your flight plan.