By coincidence I discovered that I got a ride that was not uploaded to Strava.
Then I checked the companion app: It was there, but the title is just a generic date and not the title I gave the workout and the screenshot is missing. When I try to view the activity I get an error “Activity details could not be loaded”:
Kinda similar on the web portal, I can see the ride in my activity feed, but I cannot load it. This is the activity for reference.
The log does also contains some connectivity issues. E.g.
[21:01:25] NETCLIENT:[ERROR] Curl error:  ‘Couldn’t connect to server’ for: PUT https://us-or-rly101.zwift.com/api/profiles/1424280/activities/490642630817915760?upload-to-strava=true
(I can provide the entire log file if you would like.)