Hi, I had the same problem with my Oneplus 6.
The theme is that Android Pie brings different improvements to optimize the operation of the battery, these improvements minimize the operation of the App in the background.
I have worked the following:
1.- Forget the Garmin SMARTWACTH
2.- Go to Settings OnePlus 6, specifically to applications and notifications,.... go give all permissions to the app Garmin Conect, Inluso you have to allow this app to make changes to the system.
3.- In Garmin Conect, re-link the smartwatch.
After this it works without problem.
Greetings, I hope I have helped you all.
Pablo.-
Actually, you don't need to re-link the smartwatch in Garmin Connect.
This worked for me with my FR465M and Nokia 7 Plus with latest Android 9.0 Pie update:
1) On the smartphone, go to Settings => Connected devices => Previously connected devices => Forget the Forerunner device.
2) On the Forerunner, go to Settings => Phone => Pair Phone.
3) On the smartphone, go to Settings => Connected devices => Pair new device.
5) Android will now ask a pin code. The Forerunner shows a 6 digit pin code. Enter the code on your smartphone.
Once the device is connected in Android, open Garmin Connect and it will recognize your device again.
I received during Christmas update to Android 9.0 Pie (Oneplus 5T) and the Garmin Connect app does not recognize my vivoactive3 anymore. I've tried all the tricks suggested here; removing the watch from my phone's Bluetooth devices, trying pairing from the app, gave all the permissions to the app, re-installed the app etc. Nothing works. Really upset about this.