I've had a few problems with some Bryton .fit files not being properly recognised by Strava. Its an intermittent problem where Strava can extract the summary data but not the track info, hr, cadence, etc. Some of the other services like Ride with GPS and Map My Ride have also had problems with the same files that Strava had problems with, but not always. Generally I have been able to upload the problematic files to BrytonSport and then transfer them across to Strava. I put in support requests with both Bryton Sport and Strava.
The latest advice from Strava Customer Support is that they're "working with Bryton to get the data formatted properly when it's synced to Strava."
Hopefully it will be sorted in due course.
OB