Hi,
I've been struggling with this problem and found an alternative to do a "hard reset" on the garmin (as pointed on this discussion : https://forums.garmin.com/sports-fitness/cycling/f/edge-1040-series/407822/garmin-coach-workouts-often-do-not-sync).
I share it as it might work for others...
Connected your device to a PC, navigate the device directories and search for the "GarminCoach" folder.
Delete the file on this directory. disconnect the device and force a synchronization via the mobile garmin app.
It's working fine since I did this, and even when the training session changes from one day to the other it sync correctly now.
Hope it helps.
Cheers
Ricardo Correia