Wordpress has nothing to do with it. It might be installed on the same server but phpBB totally neglects it - unless you decided to alter phpBB in such a way that it's influenced by Wordpress but in that case I cannot provide support.
Check your posts table. What is the charset and collation? It should be something like
utf8 - utf8_bin.
Also: check if the problem exists as you switch to the default prosilver style.