@Shaw, A Apologies for the delay in response and all the inconvenience caused because of the issue.
As mentioned here When you buy reserved instance you are not buying an actual virtual machine but actually buying something like license that is applied to a license. In order the reserved instance to be applied to Azure Virtual machine you must create Virtual machine with the same SKU and region as the reserved instance purchase. Also the VM needs to be in the same subscription to which the reserved instance was applied. Keep in mind that if you have more than one VM with the same parameters the reserved instance will be applied to one of them randomly. If you wish you can assign the reserved instance to specific resource group to scope it to be applied only to VMs in that resource group. If you wish to see the reserved instance resource itself you can search on the top bar reserved instance to find your reserved instance purchase.
As mentioned here When you buy a Reserved VM Instance and select Optimized for instance size flexibility, the discount coverage applies to the VM size you select. It can also apply to other VMs sizes that are in the same series instance size flexibility group which might be the use case in your scenario. To get the information regarding which Instance corresponds to which VM I have reached out to internal team to get better clarity and will keep you posted once I have an update :)
Thanks