I was able to sync it this morning by rebooting my watch and the iPhone but it won't sync now. I am considering pairing my watch with my old iPhone 6 running 12.4.8 as a temporary workaround. It is possible that Garmin Connect was broken by iOS 13.6 security fixes in which case it would likely be broken on iOS 12.4.8 as well (I upgraded last night). So one of the minor pains in upgrading.
Update: rebooting the iPhone gets the sync to working again but that's not a viable solution.
Update: rebooting the iPhone gets the sync to working again but that's not a viable solution.