The regular expressions used for search results highlighting here https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/search.php#L1063 and here https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/search.php#L1146-L1147 don't use unicode options. This leads to highlighting may miss unicode strings as \w sequence recognizes only characters with codes less than 128 via a lookup table.
Adding u option should make \w using Unicode properties to determine character types.