GPS can be dodgy even at the best of times. My phone and my car GPS both lose connection at times even on fine clear sunny days. Could be many causes none of which I'm 100% sure about. Satellite may go out of range b4 a new one comes into range.
Low power on the watch.
Signal interference from cell towers or other stronger GPS signals in the area.
Curvature of the earth may mean your route takes you out of line of sight.
Buildings may have interfered depends on where the satellite was on the horizon.
Other reasons could be:
North Koreans shot down the Satellite
Hackers hacking the GPS signals
Alien interference piggy backing on the GPS signal to co-ordinate their attack
Some idiot hit the off switch by mistake
My suggestion would be to run the same route again a couple of times at the same and different time of day and see if the same thing occurs, if it does then pick a different route and try that.
If it keeps happening no matter where you run or what time you run then perhaps get your fitbit replaced if needed.
GPS uses a bit of power so keep an eye on the battery level as low power will almost certainly impact GPS tracking
