If you've got laps turned on so you can do a longer ride, it's best to pick a ride from the Loops category.
If you don't do this, Strava will "join the dots" between the missing GPS points, and add the distance back to the start to your distance. An extreme example of this is our "Eurobike Showreel" ride which joins up pieces of rides from around the World.
Actual distance 28km - Strava distance 64,31700!!!
We have created the Loops category to solve this problem for you. They all start and finish in the same place, so no extra distance is added.
This isn't a FulGaz issue, it's how Strava and other data logging applications and websites need to work. Bike computers often suffer from data dropouts, so these applications have evolved to fill in the gaps and make everything look seamless correct.
We even tried an experiment to simulate FulGaz outdoors. We went for a ride and got the train home (with the bike computer off). When we turned it on again, Strava and TrainingPeaks drew a line back to the start. (we didn't stop it before turning it off).
Pick rides from the loops category
If you want to do repeats of a course in one session. Start the ride fresh each time.