For some reason, though the link is there, the activities do not sync over and by extension doesn’t make it to TrainingPeaks, which is linked with Garmin Connect
I think Garmin Connect is currently set up as a one way sync. Can you try downloading the .fit file from Xert and uploading to GC?
Can you sync from Strava to TP? The workouts can be synced automatically to Strava.
Record the workout on your Garmin at the same time.