$msgOptions = array(
'subject' => $txt['split'] . ': ' . strtr(Util::htmltrim(Util::htmlspecialchars($subject)), array("\r" => '', "\n" => '', "\t" => '')),
It is used, to create and save the subject, so if you changed it after you split the topic it will not change.
I'm saying this because I see at transifex that is translated as TEILEN, so I'm not sure if you changed it before or after.