The forum search engine sucks, don't use it again. It's actually working, but the result is terrible especially if you're writing quite long, it might show no results.
It's better for you to use google search or ninjastic.space.
Thank you for this, I was as confused as the OP when I first try the search bar on the uperr right, then I realized there is the "search" menu on the upper left, but I was taken back of how complicated it is.