There is a bug in the forum search software, acknowledged by the software maker. I’ll check to see if they’ve posted a fix yet. Though really, I suppose I could just replace that search box with the google search used on the rest of the site.
This latest version of the software has been full of bugs. For instance, the older versions used to let me find out which sites were linking to the MoH, but that feature no longer works because of a bug. The tradeoff to the buginess is that the software is cheap enough for me to afford.