I assumed the HereWeGo app Sailfish compatibility was broken. Playing around with it a bit, the only way I can get HereWeGo to start tracking is to run the GPSInfo app first in order to acquire the satellites.
(I also tried briefly with Sailfish OS native “PureMaps” app, and I couldn’t get a GPS lock with that either. However, I didn’t persist with PureMaps because I find HereWeGo better)