PayPal MTS has followed up on the ticket we'd opened with them about this checkout issue, confirming their Devs have now fixed the problem as of 9am MST/PDT (GMT -7) this morning.
If anyone is still getting any buyers reporting the problem behavior, please ask them to try again now. Any remaining buyers still having trouble may just need to clear their browser cache and cookies, to make sure their browser will load the current, fixed version of PayPal's checkout pages. If that doesn't resolve the issue for them, please do let us know, so we can pass that along to MTS.
Thanks to everyone for your patience and understanding while we waited on PayPal to sort out the issue at their end.