Forza horizon 4 - cross platform

Hi. I’m trying to play my son on Xbox. Via cross platform play via my gaming PC. I own the forza 4 game and my son has the game via Xbox gold ultimatesubscription. However whenever we both try and play it says that one of us has to sign out! How do I get
Microsoft to realise I am playing an owned game via pc and my son is playing a version of the game via Xbox gold ultimate.

Tried uninstalling and reinstalling on Xbox via game pass but didn’t worK

the problem appears to be two licenses for forza in one account. One is a physical license and one is via Xbox gold ultimate. Both track to same parent account shared via family sharing. Would have thought we should be able to play the game twice I.e. at the
same time with two licenses via family sharing but can’t work it out.

:)