Weird bug with planes at ~20km?

So, I was flying with a little plane I made at about 700m/s (surface) at 20km, which evaluates to Mach 2. But all of a sudden, the surface speed would change to 800m/s, despite the fact that the Mach number would stay the same. I checked the orbital speed and it gave me the same number as surface speed. The change happens at 20500. Maybe the game is just displaying orbital speed where it's not supposed to. Has anyone else experienced this issue?