CSS not appearing or functioning as expected - html

Question

I'm using django for templates. Here is the base:
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.5.0/css/bootstrap.min.css" rel="stylesheet" />
<main role="main" class="container">
<div class="pb-2 mb-2">
{% block page_header %}{% endblock page_header %}
</div>
<div class="mt-1 border-top">
{% block content %}{% endblock content %}
</div>
</main>
When the page_header div is given the class border-bottom, the border appears at the bottom of the contents instead of the page_header, not separating the two divs at all. When the content div is given the class attribute border-top, the contents are clearly separated from the page_header by a line. Why doesn't this occur with border-bottom?
page_header, plugged into the {% block page_header %}
<h2 class="d-inline">Topic: Private</h2>
<p class="ml-3 d-inline">Entries:</p>
<p class="alert-gray rounded p-1 pl-2 pr-2 d-inline border-0">2/3</p>
<a class="btn btn-gray p-2 ml-5 d-inline" href="/edit_topic/13/">
<i class="fas fa-edit"></i>
Edit topic name
</a>
<form class="ml-5 d-inline" action="/delete_topic/13/" method='post' title="Delete Topic">
<input type="hidden" name="csrfmiddlewaretoken" value="">
<button class="btn btn-red p-2" name="submit">
<i class="fas fa-minus-circle"></i>
Delete topic
</button>
</form>
<div class="btn-group dropright ml-5 d-inline" title="Visibility">
<button type="button" class="btn btn-primary btn-md rounded-sm" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
<i class="far fa-eye-slash"></i>
</button>
<div class="dropdown-menu w-10">
<form class="dropdown-item" action="/topic_visibility/13/0" method='post' title="Private">
<input type="hidden" name="csrfmiddlewaretoken" value="">
<button class="dropdown-button" name="submit">
<i class="far fa-eye-slash"></i>
</button>
</form>
<form class="dropdown-item" action="/topic_visibility/13/1" method='post' title="Public">
<input type="hidden" name="csrfmiddlewaretoken" value="">
<button class="dropdown-button" name="submit">
<i class="far fa-eye"></i>
</button>
</form>
</div>

Answer 1

It was due to an un-closed div tag.