Even though RFC 8657 does state in section 5.2 that
Domains configuring CAA records for a CA MUST NOT assume that the restrictions implied by the “accounturi” and “validationmethods” parameters are effective in the absence of explicit indication as such from that CA.
I would like you to consider failing the validation process in boulder when unrecognized parameters are present in the CAA issue and issuewild properties.
Or go all out and amend RFC 8659 so that the Issuer MUST deny the issuance of certificates if it does not understand any of the optional parameters for issue and issuewild properties.
This would, IMO, improve the user experience in a meaningful way by avoiding nasty surprises.