Same here, using Xiaomi Mi 9T with Garmin VivoActive 3. It does seem to sync when i have the app connected, but i dont receive any smartphone notifications on my watch.
I get this too - followed by the text "Android notifications have stopped unexpectedly. In order to restore notifications for your Garmin device, you will need to reboot your phone." I do nothing and things…
SOLVED the issue on my phone (mi 9t).
It was under battery saver options. I had to turn of "use adaptive battery"
Maybe your phone has a similar option, it's worth to check it out.
I too am having this issue with a OnePlus 6. It will sync data when I open the app but even when it says connected I don't get any notifications. I uninstalled and reinstalled Garmin Connect. It worked for maybe a couple hours then stopped. Very frustrated.
I'm on mi 9t too, did what you said, disabled adaptative battery and set Garmin connect to no battery economy, it didn't work. Uninstalled and installed Garmin connect again, set it to no battery restriction and it worked, but just for a few hours. Any guess?
I had the exact same thing. Reinstalled Garmin Connect, which seemed to have fixed the issue. When my phone went out of range with the watch, and reconnected again afterwards, no notifications were coming through on the watch anymore.
Xiaomi Mi5s + garmin vivoactive 3 music