"Invoice is not refundable" error when trying to refund subscription via web API

I am currently trying to refund a subscription using the API request found at https://developer.roku.com/docs/developer-program/roku-pay/roku-web-service.md#refund-subscription

The response I'm getting is

{
    "errorCode": null,
    "errorDetails": null,
    "errorMessage": "Invoice is not refundable.",
    "status": 1,
    "RefundId": null
}

But there is no indication as to why it isn't refundable. I've confirmed that the partner API Key and transactionID are valid and I'm able to validate the transaction and cancel the subscription via the API without any issues, it's only refund that's receiving this error. I've tried to refund both before and after cancelling the subscription with the same result

Things that I'm currently unclear on that might help but I haven't been able to find answers to:

• Do subscriptions have to be flagged in some way in order to be refundable?
• This transaction was done via a tester account, is it possible that test transactions cannot be refunded? (and if so how would I go about testing this functionality?)