Workout sync from Watch to iPhone is unreliable since iOS 14 update
It’s official, Apple published an article that to me explains the update had some problems and our devices may not fix themselves. It also warns of future data loss. I got bitten by the GPS bug and was not successful in fixing it by unpairing the watch, but I will follow this sequence later today once I close my rings :-) Looking closer at noise data - that's off for me and my watch isn't unlocking my Macs anymore so I'll want to restore my iPhone and watch together (first unpairing the watch, then backing up the phone, then erasing the phone and restoring / repairing)

https://support.apple.com/en-us/HT211865


If you updated to iOS 14 and watchOS 7 and are missing Activity or Health data, learn what to do to prevent future data loss.
After updating to iOS 14.0 and watchOS 7.0, you might notice:

Your
workout route maps are missing in the Fitness app on iPhone for
previous GPS-enabled workouts from your Apple Watch.
The Activity,
Heart Rate, or other health-related apps fail to launch or load data
on your Apple Watch.
The Fitness app or Health app fail to launch or
load data on your iPhone.
The Health app or Fitness app is reporting
an inaccurate amount of data storage on your iPhone.
The Activity app
is reporting an inaccurate amount of data storage on your Apple Watch.
Your environmental sound levels data or headphone audio levels data
from Apple Watch is missing in the Health app on iPhone.
Increased
battery drain on your iPhone or Apple Watch.

If you experience two or more of the above issues, follow the steps below.


Old information for context before Apple acknowledged this issue.
I had this once in the past and needed to work with Apple Support to enable a support PIN so they could see my workouts sync to the cloud and diagnose the errors. In the past, they identified something the server code could clean up and all the workouts synced within 2 days of the fix.

They walked me through one additional quick fix you didn't mention - unpair the watch and re-pair it.

Unless this is a widespread bug, the sooner you start with support, the less likely you'll lose your workouts if that's your primary goal. For a data point, I've logged dozens of workouts since upgrading all my devices on the initial day updates were released. My account hasn't needed anything, but there was a sync error in the past that caused this precise error. (In hindsight, every one of the workouts that synced were missing GPS data - so my failure wasn't initially evident as I didn't review ALL the data after seeing the initial parts correct.)
They told me if I wanted to wipe and lose all activity from the watch, I could unpair it, then back up the iPhone to the cloud and then wipe the phone from Finder, then it would all download - but since I had a lot of photos, I declined that and waited out the fix for the sync error they identified based on looking at the logs on the device. The concern was the backup to the iPhone wasn't guaranteed based on the logs as that used the same sync code as the workout sync.