Hardware and software setup as in subject line. My watch never successfully auto-syncs heartrate or activity data to the phone in the background, and every time I start the Garmin Connect app it says that the watch has disconnected and I need to reconnect. (This is made even more annoying by then having to turn off the default Move alert every time, and by Garmin sending the "tanks for registering your device" email every single time.)
A lot of Googling suggests that aggressive battery optimisations by Samsung and/or Android may be the cause of the problem, but so far as I can see there is no way to stop it.
I've tried, in the phone system settings:
* turning off "Battery -> Background usage limits -> Put unused apps to sleep"
* Adding Garmin Connect to "Never auto sleeping apps"
The second option is itself problematic: you can only mark an app as "never auto sleep" if it is set to "Optimised" in "Apps -> Garmin Connect -> Battery".
But although I keep changing this setting to Optimised, which allows me to then mark it as "never auto sleep", something mysteriously keeps changing it back to "Unrestricted", which paradoxically then removes it from "never auto sleep".
Regardless, even if I set it to Optimised and Never Auto Sleep, even then, the bluetooth connection is still lost within a few minutes. But then after a few minutes more, it is set back to Unrestricted, which removes Never Auto Sleep.
Once I reconnect, everything works OK. All other Bluetooth devices work fine for me, although I don't use anything that needs a background connection like this.
I cannot find any other options that might help control this, or any way to diagnose it.