"Search threads started by this member only"
The issue occurs when trying to search for 'Keywords', 'Posted by Member' and 'Search threads started by this member only'. In this situation only the thread titles is searched (even though the Search titles only box is un-checked). Ideally the search would query the message content as well as the thread title, but unfortunately it only searches the thread title content.
I suggest this behavior is changed so that by default the message content is searched or both the message content and thread title content is searched.
I created an add-on that forces the message content to be searched instead of the thread title content.