Hello nicolai2020,
Thank you for reaching out in Apple Support Communities. I understand that the automatic reply for emails in the Mail app, is not displaying the correct information in the subject line.
Automatic replies in the Mail app is a convenient feature that I'd like for you to enjoy. I'll be happy to help.
If you haven't tried yet, I recommend to temporarily remove the rule for automatic reply through mail preferences. Once that's done, you can re-enter the rule for automatic reply to see if that helps resolve the issue you're experiencing. If needed, here's how to access the rules in the Mail app: Reply to, forward, or redirect emails in Mail on Mac Specifically the section that says, "Automatically reply to or forward emails."
In addition, I also suggest to follow the steps outlined here: If emails aren’t displayed or look strange in Mail on Mac
Please let us know if that helps.
Kind regards.