Why 404 when querying durable task instances?

[stevendarby]

After initiating an orchestration, I'm hitting this path to list all instances:
GET /runtime/webhooks/durabletask/instances?code={code}
Where {code} is the system code, and receiving a 404 with no body.

In the logs I can see this, which indicates it's receiving the request in some form.

info: Microsoft.Azure.WebJobs.Hosting.OptionsLoggingService[0]
      ExtensionSystemOptions
      {
        "extensionName": "durabletask",
        "webhookAuthorizationLevel": "System"
      }

I get the same when specifying the specific orchestration ID too, but I could understand more why that might return a 404 as opposed to the list endpoint. What could cause this, does anyone have any ideas?

[stevendarby]

Also if I removed the code query param, I get a 401, so it's not like I'm hitting completely the wrong URL