I would probably use this HUBL instead:
{% set my_topics = blog_topics('default', 250) %}
<select id="blog-topics">
<option>Filter by topic</option>
{% for item in my_topics %}
<option value="{{ blog_tag_url(group.id, item.slug) }}">{{ item }}</option>
{% endfor %}
</select>
It seems that ‘content_topics’ might only be getting the list of tags used for the particular blog post you are currently landed on, whereas the above code retrieves the list of all used tags in blog posts. Keep in mind, I say “used” because I think the list will omit unused blog tags.