I could not reproduce the error with the verification stuff so it could be one or both of those causes, given that I redid the experiment only after logging in to geekhack, and saw it fail to find some text string that I have manually verified actually exists in that sub-community. I have to speculate that there must not have been any regression testing done on that search feature, or did, but then they stopped running regression tests:
Show Image
(https://i.stack.imgur.com/o5HAe.png)
In addition to searching in only one board, checking the "Search in topic subjects only" box further reduces the search pool.
(https://i.postimg.cc/637RxLRX/230423a-topic-only.png)
This found three topics. No idea if it's comprehensive, but better than nothing.
(https://i.postimg.cc/y6fVq4j0/230423c-topic-only.png)
Edit: Note that sometimes it helps to be a bit vague when specifying what to search for. In this case, searching for "favorite mouse" wouldn't have found #1 "What your fav mouse?"