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.
An API call in Azure can be used to get this information. Below is an example call to check AppliedScopes and BillingScopeId on the reservation:
/providers/microsoft.capacity/reservationOrders/{reseravtionOrderID}/reservations/{ReservatioID}Refer to Azure this documentation for more information: Reservation Order - List