Reservations purchased under one subscription are sometimes incorrectly displayed/listed under a different subscription in the Cost History report.
If the AppliedScopes and BillingScopeId on the reservation are different, the Cost History report will list the reservation order under the BillingScopeId subscription from Azure.
You can do an API call in Azure to get this information, like below to check AppliedScopes and BillingScopeId on the reservation.
/providers/microsoft.capacity/reservationOrders/{reseravtionOrderID}/reservations/{ReservatioID}
Refer to Azure Doc: Azure Doc