Question DKIM: How should the mailserver deal with forwarding of unsigned mails

[B_P]

Dear all,

For most of the domains hosted on my server (Ubuntu 16.04.6 and Plesk Obsidian 18.0.27, Postfix/Dovecot) I used DKIM to sign outgoing mails.
What I just observed on my server is that Plesk does not only sign outgoing mails that originate from this server but also all incoming e-mails where some forwarding (e.g. to external servers) has been set up.

Is this intended?

Best regards,

B_P