I had the same issue and contacted apple developer support and they said it was a bug in their system and then proceed to fix it. The url now shows up in my dashboard.

Hello Levon, Were you able to find a solution to this issue, I am having the same issue? I am using Firebase cloud functions to receive the endpoint. After a successful purchase the receipt is delivered client side within seconds but my server doesn't receive the call for up to 4 minutes. Meanwhile the user is left waiting for their subscription to be approved. The request is being received and processed successfully by my server within a couple seconds, but the delay in receiving the event from Apple is unacceptable. I have only tested this in the sandbox environment. Did you have these issues in production? Thanks, Jon Paul