We're trying to implement an external Auth provider for a community. It is the only auth provider that is enabled on the community, and it seems to be correctly configured, since it works as expected when using the *.force.com domain.
When accessing the community via the custom domain, clicking
Login on the community page takes you to the community login page, and does not redirect you to the provider.
When accessing the community via the default force.com subdomain, clicking
Login on the community page automatically redirects you to the auth provider login page (if you're not already logged in with that provider), or authenticates you and redirects you back to the community with a profile (if you are logged in on that provider).
Can anyone shed any light on why it works with the default domain but not the custom domain?