Hi there,
This issue happened to me a few times already and I believe it also occurred before firmware upgrade.
I was doing a training from Garmin Coach (Jeff Galloway Half-Marathon), the watch was counting down distance to the end of a particular exercise.
I was listening to some music and was curious what song was playing so I got out from the run app (long press back button) and flicked through widgets to get to the music widget.
After I checked the song title I got back to the run app. A message "GPS acquired" was briefly shown. The app was displaying this same screen as before (exercise count down) but only after a while I realised that the counter no longer moves down. To get it working I had to pause and start the activity. Only then it continued counting down.
After I realised something's wrong I verified the steps and was able to fully reproduce this issue.
Interestingly enough, when looking at the GPS track on a map after uploading finished activity there are no straight lines or odd traces (as normally seen when GPS connectivity is lost). The only indications that something went wrong are:
1) overall activity distance is shorten by the distance ran when counter didn't work
2) the lap during which this issue happened shows increased time (so for example instead of 4x 1KM laps with 6min/km pace i see only 1 lap with 24min/km pace).
F5 Saphire
SW version: 19.10 (57650b0)
GPS: 2.60