Connecting MFA account redirects to invalid URL

We are testing 3.0.0-alpha2 on a Drupal site setup with a drupal base that is not the server root.
/sitename/

After installing openid connect, with no change to settings apart from adding a new MFA, on connecting an account to MFA, the user was redirected to:
/sitename/sitename/user/1234/connected-accounts

Instead of
/sitename/user/1234/connected-accounts

On successful connect. Presumably because Url::fromUri('internal:/'.'sitename/user/1234/connected-accounts') in authenticate() in OpenIDConnectRedirectController is appending the Drupal [base].

If I force:
$destination = 'user';
(the default fallback)
in saveDestination() in OpenIDConnectSession then I no longer get the double encoding.