I just ran into this exact same issue. If you out a breakpoint in strava’s embed script, you can see the element they get when they run their query on the dom: it’s suddenly just a raw:
<div class="strava-embed-placeholder"></div>
So all the data-* attributes with real IDs you tried to enter have been stripped. These attributes are a very standard web practice. See Mozilla docs on HTMLElement/dataset… I’m guessing there’s an html sanitizer that was written without knowledge of this very safe practice. I couldn’t find the writefreely code responsible after a quick search, but I’d be happy to send a patch if it’s welcome.