Garmin rolled out GCM 3.22 for Windows Phone today. I am still testing but started to having issues with notifications again. Will report about Bluetooth stability and battery drain in some time.
I have tried everything on this thread but can't get notifications to work. Fenix 5X was up to date until 6.00 released today, GCM is 3.2.2, Lumia 950XL is on the latest fast ring (15252.0) but I get an error in GCM when trying to set smart notifications that "Smart Notifications are not fully supported on your Windows Device".
I have had notifications work in the past but not for a month or more.