Hi OPEN DES,
FCTM AO-034 P 13/20:
"If the altitude information is unreliable, the FPV and V/S are also affected. In this case, the GPS altitude, if available, is the only means to confirm whether the aircraft is maintaining level flight. When the altitude information is reliable, use the FPV."
The FPV is inertial initially, but then modified in the longer term using Alt information.