That is a problem, the $_GET['s'] should be set on both cases for sure, the htaccess rule should basically preset that. I don’t understand why though, that is going to be something in the rules. That is a very important query parameter, it basically tells WordPress that the current query is the search query, without that the override will not trigger. This is probably a bit more complicated as I thought. You may have to experiment a bit more with the .htaccess rules, or perhaps ask someone with a bit more experience on mod_rewrite rules.