Copenhagen Suborbitals has a blog-post about those limits:
By default all GPS units have some built-in limits (so-called COCOM
limits) that prevents them from providing any data if the velocity
exceeds 515 m/s at altitudes above 18 km. These artificial limits are
built into GPS receivers to prevent bad guys from using them in
missiles and other nasty stuff.
https://copenhagensuborbitals.com/gps-without-limits/