XML Attachments in notifications, weird/ non sense behaviour.

Summary:

Hi, please has anyone seen this before?

I am completely stuck, because I do not know where to look for.

In OIC gen3, I have isolated an integration to send notifications with xml attachments.

When I send an xml without xml tag, as shown:

the notification works, and is sent.

When I include this

the notification is not sent.

I have sent manually xml files as attachments to my mailbox with no issues.

Please, any explanation? I have been all day long investigating to isolate the issue, but no idea how to solve, and for me makes no sense.