The MPN ID you provided does not exist

Question

We are trying to assign our MPN ID to an App Registration. The App Registration is part of the same Tenant and Domain of an existing App Registration that has an MPN ID assigned to it.

The user account we are using to assign the MPN ID is a global admin.

From my understanding, it's possible to have multiple App Registrations using the same MPN ID, as long as the Domain/Tenant is the same and validated. Why would we still be seeing this error?