I wouldn't disagree that ideally WHMCS should fix this "illegal" behaviour.... but i've seen how long it takes WHMCS to even acknowledge that something might be illegal (they're not invoicing legal experts), let alone get around to fixing it.
if WHMCS were reactive, responsive, dynamic (none of those adjectives will ever apply to them!), then I wouldn't have spent 5+ years thinking of workarounds to resolve issues - and also to get around the pathetic slowness of WHMCS development. 🙄
it was bound to happen eventually.
you could do in in the PDF template using (based on v7.6.1 Six template)...
$pdf->Cell(0, 6, Lang::trans('invoicesdatedue') . ': ' . date('d/m/Y',strtotime('+15 days',strtotime($datecreated))), 0, 1, 'L', '1');
adjust the date format accordingly to suit your site..