When issuing a search phrase, i.e.

"Search here not working"

the operator word not is interpreted and as such the whole phrase is crippled to

"Search here - working"

Example: https://www.phpbb.com/community/search.php?keywords=%22Search+here+not+working%22

However, operator words should only apply to keywords, not phrases. When searching a phrase operators don't make sense in there, only outside the phrase (i.e. negating the whole phrase, or finding phrase and phrase).