It seems that this might be an intentional change by Garmin. My activities are working because they are recorded on a Garmin device and uploaded using the Garmin Connect app. I had a look at @Theo_Bromine’s data and his missing activities are from Zwift and @Justin_Hopkins has missing rides from Trainer Road.
I found this on a Garmin developer support forum:
Fri, 04/02/2021 - 10:48 “Activity File 3rd party upload file update” - Disappointed
We and the users are really disappointed about this change. Please consider to reenable the sync of third party activities again.
I also found this: Garmin Connect Data Is Not Syncing Over to My Third-Party App | Garmin Support
NOTE: Garmin Connect will not forward data that has been received from one third-party fitness website on to another third-party website that is also linked to your Garmin Connect account. This aligns with the policies of other third-party services and helps to prevent duplicate data from occurring across accounts.
I am going to investigate other ways of getting activities maybe via DropBox. But this is a real pain.