The callback in Purchases.GetOfferings method is never called on Amazon Fire (RevenueCat 4.0)

I am testing revenue cat 4.0 with Unity 2020.3.8f1 on fire tablet amazon but the callback of Purchases.GetOfferings is never called. With other platforms (iOS and google play) it works correctly.

I’ve already configured amazon revenuecat key, shared amazon key,set useAmazon=true and enable sandbox mode as write here: https://developer.amazon.com/docs/in-app-purchasing/drm-sample-app.html#step-6-enter-sandbox-mode

I have created a scene empty with your script “PurchasesListener.cs” (and Purchases.cs) to replicated the issue.
The callback in GetOfferings is never called either on PurchasesListener.Start() or when you pressed on the UI Button “GetOfferings”