Are you sure about that? I have a Garmin Fenix 6 and the Connect app on Android. While it's true that all data is stored locally on the watch and it doesn't need to be tethered to a phone to work, I'm pretty sure that syncing the watch to the Connect app does require the Connect app to be connected to the server. I have experienced this multiple times, after completing a hike in a remote area and I try to sync the Hike Activity to Connect it fails due to not having a cellular connection despite the watch being connected to the phone via Bluetooth.
I could swear I've also gotten data sync's while in limited connectivity situations, but to be fair could be that occasionally it was just able to phone in enough to do its thing.
Service is back up, so just testing real quick, at least on IOS , turning off wifi/cellular (not airplane mode) - my fenix at least seems to still be streaming heart rate data to the app. Not entirely sure what else will update from local data though (if anything).
