@csharpfritz first question is how long between the webhook subscribe did you leave before checking the subscriptions endpoint. I have found there can be a small delay at times.
From the looks of the url and me doing the same sort of thing, are you using an Azure Function. If so Javascript or c#?
I ask as I’m running around 1300 webhook subs and 50% of those are follower hooks that go through Azure Functions into a CosmosDB