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.
same problem here with my huawei p20 pro. very annoying!
I have the same issue with my Huawei Mate 20 Pro and Forerunner 235 - annoying! Never had this issue with Motorola Nexus 6.