This is a phenomena people have observed on Stack Overflow as well. Perhaps ironically I used regular search (“why are duplicate suggestions better than search”) and didn’t find that question on Meta Stack Overflow. But since I knew it existed, I switched to asking a question with the same query as my title and got a list of duplicates. The consensus was that emphasizing titles made the difference.
I played around with searching titles (in:title here and title: on Stack Overflow) and that’s not enough. Titles are just too short. Unless you hit on the exact words used in the title you are looking for, there’s no match. So I suspect the answer is what @JammyDodger suggests: title+body of first post. (Or title+body of question on SO.) This makes sense because when you start a topic (or ask a question) you are more likely to use phrases like other people starting a conversation than people continuing one.
Take this post. I didn’t use the phrase “similar topics” until just now because it was the context set by the first post of the topic. I also included words tangentially related to the main topic, such as “Stack Overflow”. As a topic grows in length, the odds that a side conversation will have search terms you are looking for increases too. Maybe that tangent 100 posts down is what you are looking for. My guess is that you’d be better off looking at results from just the first post in a topic, however.
You can simulate the similar topics search using the in:first search option. I don’t know if it weighs the title the same way and there might be other differences. (I haven’t looked in the code yet.) That said, the search results on your query don’t seem any better. Feels not that different from search without the option. Maybe adding weight to the topic matters and there should be an option to simulate the similar topic search?
I’m also curious if a search that included the original topic post, title and any post marked as a solution would provide better results.
I should note I found this topic via regular search, but I was looking for how to suppress the “similar topic” prompts. I’m mostly using Discourse as a blog and I don’t mind writing about similar things from time to time. Starting a topic asking my question got me my answer.