Https://forum.urbandroid.org/t/garmin-cant-connect-with-the-app/4909

Garmin VIVOACTIVE 3

Everything worked as you said until Sept. of this year (2020). There was a change in the software which changed the info on the watch from white to brown.
This update broke the connection from “starting the app on the watch then opening up the sleep app on the phone”
Further more, there is no way to stop the app on the watch!!! The only way is to shut the watch then reboot!!!