Flutter: Auth0Web.loginWithRedirect ignores audience parameter

Event though I set the audience parameter in my call to Auth0Web.loginWithRedirect when the framework does a token exchange by posting to oauth/token the payload does not include the audience parameter that I set. Given that, I get back an opaque token and not a JWT - as the documentation on oauth/token says it will do.

Why would framework ignore the audience parameter? How can I debug this? (and yes the sample app does the same thing).