Right now, subscriptions purchased on traditional have only ownership for the partner.
However, the customer cannot see it, until it has not been assigned Collaborator or Owner role in that subscription. According to https://docs.microsoft.com/en-en/azure/role-based-access-control/elevate-access-global-admin , it can be regained elevating the access of the customer tenant owner.
Login with Global Admin user for the customer tenant.
1. With previous conditions, as customer has not been assigned the subscription, he doesn't have access to it.
2. Global admin can elevate his access to regain it. To do it, go to AAD → Properties → Access management for Azure resources and click on yes.
3. Logout and login
4. Now you can see the subscriptions that you didn't have access
6. Add the owner for the admin domain
7. You can remove now the elevate access permissions. As you have now access to the subscription.