The value for policy.authority-uri contains characters that are not allowed in a simple token, so it must be in quotes to be a quoted-string (https://www.rfc-editor.org/rfc/rfc2045#section-5.1 contains the definition for value which is used by ARH here).
ref: https://github.com/lieser/dkim_verifier/issues/242#issuecomment-1679395783
The value for policy.authority-uri contains characters that are not allowed in a simple token, so it must be in quotes to be a quoted-string (https://www.rfc-editor.org/rfc/rfc2045#section-5.1 contains the definition for value which is used by ARH here).