"Your account is temporarily unavailable" in app review

I've submitted by app for review with an IAP for the first time.

I have managed to successfully test the IAP myself, in a debug build on device both with a synced StoreKit config file and using the sandbox, as well as with a TestFlight release build with a sandbox account.

My app is being rejected however, and the screenshot attached to the rejection shows the error

Your account is temporarily unavailable. Try again later

[Environment: Sandbox]

The initial rejected supplied the below as next steps

When validating receipts on your server, your server needs to be able to handle a production-signed app getting its receipts from Apple’s test environment. The recommended approach is for your production server to always validate receipts against the production App Store first. If validation fails with the error code "Sandbox receipt used in production," you should validate against the test environment instead.

I'm not even attempting receipt validation through the app, which I know isn't advised but also validation isn't mandatory as far as I'm aware.

Could this possibly be an issue with the test devices? I'm not sure how else I can debug this

Update:

I replied to the reviewer with videos of me completing the purchase in sandbox and suggested again this could be an issue on their end.

I received the reply

Thank you for making these changes. We will continue the review, and we will notify you if there are any further issues.

My app was then approved despite the fact I've made no changes. I assume this must have been an issue with testing?

Some people complain about Reviews. You illustrate that by providing concincing information, they listen.

Thanks for feedback.

"Your account is temporarily unavailable" in app review
 
 
Q