iOS update 13.1.3 is causing activity sync issues within Garmin Connect Mobile App.

Solution: I suggest using Garmin Express until there is an update released from Garmin.

Garmin is aware of this issue (https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-mobile-ios/193204/attn-ios-users-with-issues-after-updating-to-ios-13)

Update Nov 5:

iOS v13.2 appears to be allowing Sync. I disconnected the Garmin watch in the Apple iPhone Setting>Bluetooth menu, then re-paired the watch using Garmin Connect.