Can't configure GPS through Mission Planner

Have latest release Rover firmware installed on a Pixhawk 2 and HERE GPS.

Changes to the GNSS parameters don’t seem to propagate through to the GPS:

GPS_GNSS_MODE - changing this has no effect

I configured it for GPS+GLONASS+QZSS+SBAS in uCentre.
Shut down.
Reconnected to MP. Set “MODE” to 1 - GPS - no SBAS.
Connected to uCentre.
Still configured for all the constellations.

Any ideas what I’m doing wrong?