Please reach out to us directly via support.garmin.com for assistance. We will need to gather logs and screenshots from you to escalate this.
I have this issue as well, except NO syncing is ever done, whether through Garmin Express on Windows or the mobile app on Android. I tried virtually everything I could and all that on a Vivoactive HR brand new returned from customer service in France. It's driving me absolutely nuts.
The weirdest thing is : on Garmin Web, on the device infos it says "Synced august 14th" which corresponds to the l ast time I used it like 2 years ago. Even if I totally remove the device from my account and add it again (also whether it's on mobile and / or Windows, tried both, separately, at the same time, etc.), guess what ? It still says "Synced august 14th". It's like my account and VIVO HR pairing has been stuck for years and just can't get out of. Mental.