No, it's a MyBB bug,
which I reported here, but being the honest bastards the devs are, they deleted it. (they forgot to remove the hanging link
here though)
It's due to their stupid workaround for mb_substr - look in inc/functions_compat.php and delete the mb_substr function - it doesn't properly handle negative arguments.
BTW, it's also the cause for the "Unknown Location" issues in the Who's Online that so many people report in the support forum. Despite what rubbish they say, it's not a server issue, but rather, a bug in their code.
I'm too lazy to fix it here, as it's not a big issue.
But thanks for reporting regardless