APIM dev portal test console OAuth: wrong aud claim

Question

We have just set up an OAuth server, for using the developer portal to test API calls to protected backends, as per recipe

https://learn.microsoft.com/en-us/azure/api-management/api-management-howto-oauth2?WT.mc_id=Portal-Microsoft_Azure_ApiManagement#security-considerations

Our backend will use App roles, no user sign-in, so in the default scope field in the OAuth config, I put the application ID URI in the form api://<Backend Application (client) ID>

Upon calling the API from the test portal, we get a 401.71 error, and upon inspection the audience claim in the token is mostly zeros:

"aud": "00000002-0000-0000-c000-000000000000",

Does anyone have any clues to why this is happening?