[Paul M]

There is no bug, you must register any variables used in the template, including those in the conditionals (certain ones like $show are auto available).

In you case, you must register both $post and $thread (in blue).

$arp_var = "{vb:raw thread.title}";

$templater = vB_Template::create('ARP_TEMPLATE');
$templater->register('arp_var', $arp_var);
$templater->register('post', $post);
$templater->register('thread', $thread);
$template_hook["postbit_end"] .= $templater->render();

Also, I dont know what the line is red is for, but all you will output is the text {vb:raw thread.title}