It appears something is wrong with the date selection and sort generally - here I filtered for posts AFTER Sep 15, and you can see the results include Sep 7 and so on:
Let me ask around the dev’s forum to see how we should be searching for this purpose. I tried the search and i did even worse than you- only one results returned.
Interestingly I looked at the search results from these two links and the results seem to be correct now. Maybe the search index takes some time to get it right?
For this, however, its only returning the latest of your posts from each WFD discussion you participated.
I posed the question in the dev support forum a few days ago but didn’t hear back. Let me post the updated, narrowed question on why the results only return one post per forum.