BG770: QGPSLOC incorrectly reports zero altitude

At a known altitude of 55m, the module reports this:
+QGPSLOC: 030419.000,37.57214,-122.36150,1.7,0.0,3,139.10,0.0,0.0,110823,6
and the related GNSS sentence is this:

Any ideas as to why the QGPSLOC altitude field is reported as zero?
Note that the HDOP is 1.7, the same value in both.
Altitude is the next field in both, 39.6 in GPGGA but 0.0 in QGPSLOC