Hi,
first of all, this behavior is not the default because adding missing headers is technically against RFC. Postfix is designed to preserve the original message as received.
Second, automatically adding headers can break existing cryptographic signatures (DKIM, ARC, S/MIME, etc.) unless you re-sign the message afterward.
That said, in your specific case (receiving messages from an HP printer) it’s very unlikely those messages are signed, so enabling always_add_missing_headers = yes is a safe and practical solution. Just make sure other mail flows (especially those involving signed mail) are not affected.