There seems to be a bug in one of the plugins.
Plugin:
2. Add class to postbit postbit_display_start
The start of the plugin should read something like this to have the correct behaviour:
PHP Code:
if ($this->registry->options['ophilite_enabled'])
{
$GLOBALS['threadstarteruserid'] = $thread['postuserid'];
if ($GLOBALS['threadstarteruserid'] == $post['userid'])
(etc...)
The approach you took made posts highlight not when they belonged to the thread starter, but when they belonged to the first poster on that page (a thread may, as you know, have many pages).