Product Key Usage and 3.1.3(b)

We have an iOS app in the store that have been available for almost two years. This app is also available on macOS and Windows (and has been available on those platforms for many years before the iOS app was released).

On macOS and Windows, users are given a product key in order to license the application (either purchased directly from us, or given to them by their institution). Depending on the purchase, the product key could be a subscription or a lifetime license.

When we released the iOS app, we made the app "free" as in, there was no cost to install it from the store. On first run, the user is presented with in-app purchasing options, and also given the option to type in a product key if they have it. We are pretty much the definition of "3.1.3(b) - Multiplatform Services." We've had a couple issues in the past getting the app through review because of the product key option, but it's always been a simple matter of pointing out that we are complying with 3.1.3(b).

For our most recent release, we were once again rejected for providing the product key option. However, after we pointed out that 3.1.3(b) allows unlocking based on purchases on a different platform, we were unilaterally told that product keys were not allowed. We had several back-and-forth interactions with the reviewer in an attempt to clarify why 3.1.3(b) doesn't apply, but we were only told that product keys were not allowed.

Since this release is a bug fix only, they agreed to release it as-is but said we would need to address the issue in our next release. We have no intention of removing the product key unlock and are expecting our next release (a new feature release) to be rejected and then force us to appeal the decision.

Has something changed recently that limits the ability to use product keys that were purchased on separate platforms? I've read through all of the relevant sections and I don't see anything materially different from when we first released the app 2 years ago. Is this just a reviewer who doesn't understand their policies? Has anyone gone through anything similar and managed to successfully appeal the decision?