Dear Maintainer,

   * What led up to the situation?

I use multiple oauth2 passdbs, one for each domain, using password grant.

Therefor, I use `username_filter = "*@example.org` to only check e.g.
user@example.org against this passdb, and not user@example.com.

   * What exactly did you do (or not do) that was effective (or
     ineffective)?

For this to work, `auth_username_format` has to be `%Lu`, because `%Ln` would
remove the domain and break the `username_filter`, so that no passdb would
match.

So the domain is used for finding the correct passdb. But the OAuth provider
expects the username to be just the username without domain, which is why I set
`username_format = %Ln` inside the oauth2 passdb configuration.

   * What was the outcome of this action?

This does not achieve the expected result. The OAuth provider is still queried
with the whole user@example.org as username.

   * What outcome did you expect instead?

The expected result would be, that (after the correct passdb was found based on
the domain) the username is translated according to the `username_format`
defined inside the passdb before the password grant request is sent.