Sharing GoodNotes 6 Premium in Family not working

I have paid one time purchase for Good Notes 6 on App Store and enabled family sharing. My son (who is a on Family Group) is still being asked to "upgrade to premium account" when he signs in to Goodnotes on his phone - his account remains "Fremium". Good notes support says sharing is available and any issues with sharing is an Apple problem. Any advice?

I have tried some of the suggestions on the Goodnotes support site without success [Share your premium Goodnotes 6 with your family – Goodnotes Support]