Adding additional roles on registration

You check the role not for the created user, only the currently logged in user. Try this way:

add_action( 'user_register', 'se333727_additional_roles' );

function se333727_additional_roles( $user_id )
{
    $new_user = get_user_by( 'ID', $user_id );
    if ( in_array( 'um_dueling-pianist', (array)$new_user->roles ) ) {
        $new_user->add_role('pianist');
    } 
}