The softlink goes away when the RPi is rebooted. If it is recreated immediately after reboot, all 3 platforms were able to respond with data to 'gpsmon /dev/gps'. The Astroberry was also able to report GPS data with the Indi Control Panel. But the 2 Stellarmate platforms did not report GPS data in the Indi Control Panel.
This behavior has been consistently confirmed over multiple reboot cycles.