No solution, sorry, but it sounds like my issue (fenix 6). Workouts are displayed and current hr/steps/body battery but there is no history. There are a couple of blips in some graphs. Maybe from when I have resetted something. Started around a week ago. Haven't touched any settings in years. There might have been some update around the time it stopped.
Removing and adding the device back helped.
That's exactly what was advised in the previously linked article. It could mean you have a duplicate / phantom Garmin Connect account. It may happen again, so you better try finding out what the other account is, and deleting it. Search for your email addresses and aliases on the Connection page: