EM7455 NMEA GPVTG true tracking degrees is different from AT!GPSLOC?

The GPS is at rest. So the true track degree must be zero. If I use at!gpsloc? the output is correct. But the $GPVTG sentence has the wrong value. It can be 79.3 or some other value.

$GPVTG,79.3,T,87.1,M,0.0,N,0.0,K,A*20