Check from Sports Tracker Settings if there is some suggestions.
Unfortunately most Android phones have their own Battery saving mechanism to close "unnecessary" tasks to run on the background. This cause the GPS to shut down by the phone mechanisms while the screen is locked.
You should be able to fix the problem by changing power saving settings. These settings vary between phone models and OS versions but it should go somewhat like this with most common device brands.
Generally for most this should help:
1. Turn off System Settings > Apps > Gear Icon > Special Access > Battery Optimization.
2. System Settings > Device management > Battery > Sports Tracker > enable/allow app to work on the background
3. System Settings > Device management > Battery > Google Maps > enable/allow app to work on the background