They are setting aside those ballots. They are not mixed in the other ballots received on or before Election Day. PA is essentially treating them as provisional ballots, there should not too many of these. Also the legal challenge for this was already made prior to the election and the SCOTUS already ruled in favor of the State. PA is afraid that it may be challenged again though so they making sure to take extra precaution just in case. Again, there likely are going to be enough of these ballots to ultimately matter.