This problem was previously reported long ago in http://tracker.phpbb.com/browse/PHPBB3-6611 but it's been reported by one of my forum members. Briefly, the returned URL for a search is not parsed correctly if posted without modification. For example:
If posted as shown above, the magic URL code incorrectly parses it as a clickable link. See the attached screenshot; notice how the query parameters after the "" are ignored. If the returned "" in the URL is encoded as %5B%5D, the parser treats it correctly. The returned search URL should either omit the "" or encode it so it's recognized by the phpBB parser as a valid URL w/parameters.