Adding activity in a section in Moodle - not saving the section - php

Question

So I've been looking for 2 days into this problem, I can't find either where the code exists, or how to edit/fix it.
What is happening is that when you click "Add Activity or Resource" while editing a course, it adds the activity or resource, but always into section 0. It doesn't save which section I've clicked from, and when the redirect to course/modedit.php happens, the URL shows section=0, no matter if from section 1 or 10. Further to this, moving the activity into other sections also does not save - it simply re-orders the activity to the last one in section 0.
I am unsure as to which code is causing this issue. I need some help or guidance on where to look to begin debugging and fixing this.
I also can't be sure when this issue first appeared, as I have modified a few files in between while developing this site. The only one that may be relevant (from what I can surmise), and may help give some clue for anyone better informed than I is the following:
protected function section_header($section, $course, $onsectionpage, $sectionreturn=null) {
global $PAGE;
$o = '';
$currenttext = '';
$sectionstyle = '';
if ($section->section != 0) {
// Only in the non-general sections.
if (!$section->visible) {
$sectionstyle = ' hidden';
} else if (course_get_format($course)->is_section_current($section)) {
$sectionstyle = ' current';
}
}
$o.= html_writer::start_tag('li', array('class' => 'section main'.$sectionstyle));
// Create a span that contains the section title to be used to create the keyboard section move menu.
$o .= html_writer::tag('span', $this->section_title($section, $course), array('class' => 'hidden sectionname'));
$leftcontent = $this->section_left_content($section, $course, $onsectionpage);
$o.= html_writer::tag('div', $leftcontent, array('class' => 'left side'));
$rightcontent = $this->section_right_content($section, $course, $onsectionpage);
$o.= html_writer::tag('div', $rightcontent, array('class' => 'right side'));
$sectionname = html_writer::tag('span', $this->section_title($section, $course));
$o.= html_writer::start_tag('a', array('href' => '#section-'.$section->section, 'class' => 'title'));
$o.= $this->output->heading($sectionname, 3, 'sectionname' . $classes);
$o.= html_writer::end_tag('a');
$o.= html_writer::start_tag('div', array('id' => 'section-'.$section->section, 'class' => 'content'));
// When not on a section page, we display the section titles except the general section if null
$hasnamenotsecpg = (!$onsectionpage && ($section->section != 0 || !is_null($section->name)));
// When on a section page, we only display the general section title, if title is not the default one
$hasnamesecpg = ($onsectionpage && ($section->section == 0 && !is_null($section->name)));
$classes = ' accesshide';
if ($hasnamenotsecpg || $hasnamesecpg) {
$classes = '';
}
$o.= html_writer::start_tag('div', array('class' => 'summary'));
$o.= $this->format_summary_text($section);
$context = context_course::instance($course->id);
$o .= $this->section_availability_message($section, has_capability('moodle/course:viewhiddensections', $context));
return $o;
}
That is my rework of the layout - allowing for a simple Jquery accordian to show/hide the sections. The Jquery is:
$(document).ready(function() {
function close_accordion_section() {
$(".topics .title").removeClass("active");
$(".topics .content").slideUp(300).removeClass("open");
$("#section-0").stop();
}
$(".title").click(function(e) {
// Grab current anchor value
var currentAttrValue = $(this).attr("href");
if($(e.target).is(".active")) {
close_accordion_section();
}else {
close_accordion_section();
// Add active class to section title
$(this).addClass("active");
// Open up the hidden content panel
$(".topics " + currentAttrValue).slideDown(300).addClass("open");
}
e.preventDefault();
});
});
I have edited nothing else in relation to the course page, but that doesn't mean the issue might not be elsewhere.
Sorry for the wall of text, but please could someone help me find why the Activity isn't adding to the right section, nor allowing me to move into the right section afterwards? I'd really appreciate it.