What happens when you forward an email?

Forwarding creates a brand new message that quotes or embeds the original content. The original From, To, Message ID, and Received fields are no longer active headers. They appear only as part of the quoted body or a formatted summary.

When you forward, the new message uses your own From address and your own authentication. Any original DKIM signature will fail for the forwarded copy because the body has changed. Think of it as loading cargo from one ship to another. The new vessel has its own papers and its own responsibility.