Search bug?: not working as expected

Hi,

I am wondering why a search for "Negative lab Pro" does not return my created threads if I click "search titles only", even though the titles have exactly that name. If I deactivated "search titles only" then those two threads can be found.

The threads:
https://p-v.club/threads/negative-lap-pro-lightroom-plugin-win-mac.57816/#post-323267
https://p-v.club/threads/negative-lap-pro-lightroom-plugin-win-mac.56883/#post-316582

The search within titles:
https://p-v.club/search/7650076/?q=negative+lab+pro&o=relevance&c[title_only]=1

The search without title restriction:
https://p-v.club/search/7650072/?q=negative+lab+pro&o=relevance

What am I doing wrong?