When the user_update event is triggered, but $arg['is_admin'] is not set, a user loses his admin role. This is expected behaviour normally, since these are both true if a user is removed as a group admin. However, when adding a user to a group who is already a member (due to missing checks in og/users/%node/add_user) the user role is removed for the admin, but he will still be listed as owner and group manager in other OG overviews.

The only circumstance when the user admin role should be removed is if is_admin is both set, and 0.