Enabling DKIM stops list-unsubscribe headers being added to emails

[Mart]

I've had a long-standing issue with list-unsubscribe headers not appearing on my Production site but I'm having to migrate servers so I've used the opportunity to troubleshoot on the new setup and I believe enabling XenForo to sign emails with DKIM stops the headers appearing. Is that expected behaviour? I've done a lot of searching/research and it doesn't appear to be expected.

Testing:

PHP built-in email system (server set up with Postfix)
Email options set to yes (with unsubscribe...

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