That is because the heading links do not include the path to the topic. For example
<a name="overview" href="#overview">. The Chat Integration plugin is attempting to convert relative links into absolute links by prepending the forum’s root URL to any links in the post, but that is not going to work for the header links because the path to the topic is missing.
A possible solution would be for us to add the path to the topic to the TOC
href element. For example, instead of setting
<a name="overview" href="#overview">, the
href element would be set to something like