I would say it should still function as a genealogical record for a number of reasons, particularly as a useful medical record. If unknown, that should be specified - or include a reference to their anonymous medical records.
There could be another field for adoptive second parent at birth, if this is necessary. Otherwise I can see how it might cause problems for the adoptive parent in the event of a divorce. Although my understanding is this is already a formalised process, just different paperwork.