Sign out not working properly in OpenID Connect
The openid_connect module is great for authenticating/adding users via external identity providers. However, it does not provide single sign out capabilities.

You will have to implement them yourself.

A viable way of doing so is to implement hook_openid_connect_post_authorize() to add information about the used identity provider/tokens to the user session/database, and e.g. hook_user_logout() to invoking the sign-out endpoint of your identity provider, if the above saved information is present when the user logs off your Drupal.