The "forum search" function has been pretty rubbishy for years (maybe always)
It doesn't seem to make much difference whether you limit searches to particular boards or select "most relevant"/"most recent etc.
However, if you put your search term in inverted commas "---" you gat fewer responses and the phrase or word is highlighted in yellow. For example, I put "search for a particular subject" in the search and I received just one response: your message.
Good luck - and do let us know how your get on!
DAvid