It is for me too.Either it was the default or I unknowingly made it so.
Threadloom is always checked when I click on search button.
We've been aware of this, and some other bugs in XF search with member's posts not all showing in their profiles. We're trying to track-down the cause so we can hopefully find a solution.but does not work with the Xenforo search selected. I can live with this, so it's not a problem for me.