I must admit I had this problem the other day when searching for 'Beta 99' and found it frustrating. I've just altered the 'Common Word Threshold' setting on the Administrative Control Panel from "15%" to "0" meaning it is now disabled. However, this hasn't had any effect as the phrase 'Beta 99' cannot be found nor can 'Beta' on its own.
Mazeteam, our technical support co-ordinator, will hopefully be able to look into carrying out the instructions as written on this website:
http://tracker.phpbb.com/browse/PHPBB3-7847