I just got a Vivoactive HR two days ago, and I'm having issues with notifications (Lumia 950 on current release build 14393.576).
A few minutes after I paired the device with my phone, I received a phone call and got an alert on my Vivoactive HR. After that, I didn't receive any further alerts for calls, emails or texts.
This morning, I did a chat with Garmin support. The agent walked me through resetting the device and uninstalling/reinstalling the Garmin Connect app on my phone. After I was done, pairing the device with my phone again, I received an email notification on the Vivoactive HR. Then, no further alerts for the past two hours.
At one point, I ignored a phone call. I didn't get an notification on the Garmin device, but when I checked the notification widget, it listed a missed phone call. Occasionally, I receive an alert on my wrist, but when I look, it just shows "Phone Disconnected."
As far as syncing data from the Vivoactive to the phone, there is no issue. It automatically syncs each time I open the Connect Mobile app.