Scenario
As your signature is using Custom Fonts/Background image, it is rendered as a bitmap.
You notice that on reply or forward emails, the signature becomes an attachment, or it appears to be removed from the email.
Reason
Some mail clients, such as Apple Mail, can cause Embedded Images to be stripped from the signature template and become attachments.
Unfortunately, this also applies to signature or text elements that have been rendered as a bitmap - irrespective of whether you are using custom fonts or background images.
Resolution
To resolve such issues, we recommend using a font stack in the signature template - this will allow you to use a custom font with fallback fonts next to it for the mail client to choose the next one available in the list.
For more information, please see Using custom fonts without rendering as bitmaps.
As a workaround for the background image - substitute the background image as a banner image or use a background colour and split the image into different parts.