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 );