Family Sharing. Apple Music (Family). Newly added Family Member hasn‘t got Access.

We are a family of 4. I am the organiser (father). I ordered and pay for Apple Music (Family) for years now. My wife and 2 kids enjoy the shared Music Subscription. No problem so far. I recently added my own father (the grandpa) to the „Family“ within my Apple Account (of course he agreed on his side). On my iPhone, under (translated… my iOS is in french) „settings-OWN_NAME-family sharing-GRANDPAS_NAME-subscriptions“, the screen states that he has access to my Apple Music Sub. Now, repeating the same procedure on his iPhone, the page shows nothing, it‘s empty (accordingly the Music App doesn‘t recognise any running subscription). I already logged him out of his Apple Account and logged In again, without success. I have to stress that I am living in Austria, and am logged in the corresponding App Store, and he is logged in a french App Store. Could this be the reason for the malfunction? Any other hint?