Hi Bloc, I guess there's something to do with that code.
I found it in TPmodules.php, on /Sources.
}
// promoting topics
elseif($tpsub=='publish')
{
if(!isset($_GET['t']))
redirectexit('action=forum');
$t = is_numeric($_GET['t']) ? $_GET['t'] : 0;
if(empty($t))
redirectexit('action=forum');
isAllowedTo('tp_settings');
$existing = explode(",",$context['TPortal']['frontpage_topics']);
if(in_array($t,$existing))
unset($existing[array_search($t, $existing)]);
else
$existing[] = $t;
$newstring = implode(",",$existing);
if(substr($newstring,0,1)==',')
$newstring = substr($newstring,1);
tp_query("UPDATE " . $tp_prefix . "settings SET value = '" . $newstring . "' WHERE name='frontpage_topics'", __FILE__, __LINE__);
redirectexit('topic='. $t . '.0');
}