We don't know if the OP correctly identified the property being appraised, if they made an additional HC ( additional HC is not allowed anyway on 1004 ) or if they made an HC to exclude the 13-acre parcel on the legal description. Either way, the OP can be in trouble on this, what the client wants will actually save them from that.
While technically, what parcels are on a deed may not be an appraisal issue, an appraiser can add additional work to the SOW that is relevant to the assignment.
True story - early in my career, I listened to a CG give poor advice on this board and lost a great client over it, a loss that probably amounted to over 100k or more in business. I stil see, to this day, see CG appriasers jumping in on GSE work saying inane things like use a narrative report ( they'll kick it back) use a GP form ( they'll kick it back ) etc.
If the client was asking for something fraudulent or that would change the value or anything like that, of course, the advice is to refuse. But in this case, all the client is asking for is a simple make-it subject to putting each piece on its own deed. What, exactly, is the adverse appraisal issue with doing that?