Aha, it's because the extension to XenForo_Model_Thread grabs the avatar information for the last poster over the top of the thread starter. So you end up with user_id / username of the thread starter and avatar_date / gravatar of the replier.
If you find:
PHP:
$selectFields .= ',
user.avatar_date, user.gravatar';
In WidgetFramework/Extend/Model/Thread.php (it's around line 57) and replace it with:
PHP:
$selectFields .= ',
user.avatar_date, user.gravatar, user.user_id, user.username';
Then it'll cause it to put the last poster user_id / username over the top of the thread starter as well.