The only possible cause i can come up with, is because the hook ( forumdisplay_thread_end ) is too near the eval of the thread info.
Maybe a dev can answer this question, when a hook is called does it execute all the code that comes with the hookcall, before it continues with the code below the hook ? =/
$plugins->run_hooks("forumdisplay_thread_end");
eval("\$threads .= \"".$templates->get("forumdisplay_thread")."\";");
You could try to move the hook upwards.Maybe a dev can answer this question, when a hook is called does it execute all the code that comes with the hookcall, before it continues with the code below the hook ? =/