UPDATE:
If I select one of the ebike activities that isn't showing up and change the map from standard to dirt it then shows up ok.
If I change the same activity's map back to standard it still shows up.
Obviously I'd rather not go through all my activities and carry out this task, but this might be a clue as to why it is not working properly.
I saw a similar post that indicated their maps were marked as hidden which had caused this, but all my maps are visible.

Sometime in the last week mid-July, all my e-bike activities have disappeared from the web "My Routes / New --> "Personal Heatmap". My conventional rides still display for the most part. On the Strava App on my phone, the "My Heatmap" which has "All Sports, All-Time" option, it does show the e-bike activities. I am feeling frustration with my e-bike activities disappearing from the web route builder. I use that information to plan routes going in new places I haven't been before. I do notice on my phone in the Strava app that the "My Heatmap" in the route builder still shows all my activities including e-bike

Yes, I'm seeing the same. I think my original problem got fixed in a major overhaul that was done to the maps section so I'm happy with that bit now. But I agree, it is frustrating when planning a new route and you can't see the sections you've already done. I feel the developers are continually tweaking things, which is in some ways a good thing as new features are added. A bit annoying that some parts get broken for a while in the process!