Hello Daniel
Can you please provide more details about how the issue is occurring?, please provide a step by step guide to duplicate the issue, starting with how the ticket was created (you already mentioned that it happens with tickets opened by email, but please be more specific, for example if there is a template being used or if you are using a default subject, etc) and how the attachment was added to it. Screenshots of the issue would be helpful as well.
I tested in a local environment (14.1.04) by creating a ticket from an email attaching another email to it and the creation was successful along with the attachment which can be downloaded with the correct format. Also if I add the email as an attachment is uploaded correctly and it can be downloaded in the same way.
Best Regards,
Ruben Reyes