Hi,
I'm not able to reproduce the problem on my end with several websites.
Note that ORDER_END_MESSAGE is not a translation constant. It's an email HTML tag. So it's normal you don't find it in the translation file.
At the end of the "HTML" section of the order admin notification (when you edit it via the menu System>Emails), you have that tag:
{TXT:ORDER_END_MESSAGE}
That tag is automatically removed by the system when the email is generated.
What you're describing indicates that either that tag was changed to for example ORDER_END_MESSAGE in the HTML section, or that there is a bug when replacing empty tags, but only in some cases because otherwise I would have the issue too.
The simplest solution in the short term is to remove that text / tag from the HTML section of the email. It doesn't have any purpose for that email anyways.