Just in case you come in some similar situation ...
I had "auto-update" enabled in the Play Store settings on my android phone, so a few days ago the Garmin Connect Mobile app had been updated automatically. So far so good, wasn't for the first time though. But this time this version did not allow re-connection of my Garmin Fenix 3 with the app once the connection was lost (phone off or just out of reach ...). I could not find any other way (like off/on BT or rebooting phone/watch) then going through the process of re-pairing the device :(
But previous version was working just fine and could handle this situation without problems.
So what to do? Either live with it ... yeah could, maybe next update comes soon and will fix the regression. Also I have seen some app that does the re-pairing of the device for you ... well I have not tested it but hard to believe it is much faster than doing it manually. So I thought it might be the best to just install the previous version at it was just working right.
Well pretty easy actually you just go to a site like: https://www.apkmirror.com/ search for the connect app and download the previous version as .apk file.
Then push to your phone (well if you browser to the site via phone you can skip this of course) eg. via adb push. I removed (aka deinstalled) connect mobile app before (not sure if it would be problematic to keep old version ... maybe not) then enable installation of app from unknown sources in the developer settings on the phone. Browse to the .apk file and press "install" button.
Done open app and login, pair your device and you're good to go.
For now I also disabled "auto update" ;)