@Alessio RosaThank you for posting this in Microsoft Q&A.
As I understand you moved the subscription to another tenant earlier and moved it back to your old tenant. Now you are unable to access the subscription.
As you mentioned you are the Global Admin in both the directories.
To move the subscription to another directory you need to have Owner permission on the subscription. Global admin is a role with permission which are defined within the directory. And Owner role is set of permissions which is assigned on a subscription
As per process, once you move the subscription to another directory you will lose the RBAC roles which were assigned earlier on the subscription.
As per our public document when you move the subscription all role assignments and custom roles in Azure role-based access control (Azure RBAC) are permanently deleted from the source directory and are not transferred to the target directory.
In your case you will have to contact the owner of subscription to get role assigned as per your requirement.
Let us know if you have any further questions.
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