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.
Thanks Trux, i followed diligently and hoped something would provide a solution. I'd already done most things but hadn't disabled app overlays, locked the app or cleared the cache. Alas after a reboot the problem returned. What was useful however was the point about simplying turning notification off and then back on even thought is shows as enabled. This small action solves the issue. Annoying that it needs doing every day but at least it's a quick fix.
I also had the problem where the phone would beep if the app was active, but not otherwise. Connect had all app permissions, so it was quite a puzzler. Some of the settings mentioned in older articles didn't seem to exist anymore. This article led me in the right direction, but I wanted to share my specific steps for my ~2019 Android phone.
I selected settings >> apps >> Connect >> battery >> optimize battery usage.
Then, I had to choose "all" apps instead of "apps not optimized". Over 400 things showed up in this list, I scrolled way down to Connect, and un-selected this option for Connect. That was the magic setting to fix "find my phone" from my watch.