add_settings_section() order

You can adjust the priority of dr_issue_settings_init() so that it fires after the default of 10. This should make it appear after the existing settings on the same settings page.

function dr_issue_settings_init() {

    add_settings_section(
        'dr_issue_section', //$id
        __( 'Issue settings' ), //title
        'dr_issue_section_callback', //callback
        'mytheme_settings' // page
    );

}
add_action( 'admin_init', 'dr_issue_settings_init', 100 );