Thanks for your prompt reply.
Let me explain details
GPS data blocked according to “SIM_GPS_BLACK” parameter. So, the drone had no local position info.
QGC shown the altitude was 66.9 meters which was last altitude data from GPS, but reliable altitude source was only one, barometer sensor in this situation.
VFR_HUD MAVLink messages correctly reported altitude as you see.
To sum up, it will be nice that QGC gets altitude from VFR_HUD/ALTITUDE when the drone loses GPS signals.
This is because QGC kept displaying the 66.9 meters even through the drone decreased altitude and then landed on the ground.