I have Garmin Connect Version 5.16 on iPhone 15pro iOS 18.6.2 (22g100) Garmin Instinct Solar 19.10
Many times the green dot who marks an active Bluetooth connection from the app to the watch goes away.
What I do is: Close Carmin Connect and restart the app. Then the green dot is there again and the watch is syncing to the app again. This connection will work for a certain time and looses the connection again.