Can't get appAccountToken sent from react-native-iap in App Store Notification v2 + Firebase function

Question

I'm working on auto-renewable subscription using react-native-iap (mobile client) and handling subscription update via firebase function from App Store Server Notification v2

In client side, I sent using the requestSubscription function

requestSubscription({
  sku: <MY_SKU>,
  appAccountToken: <MY_APP_ACCOUNT_ID>
})

Then on Firebase function, I setup a listener to App Store Server Notifications (Sandbox URL)

In the listener, I fetch the requestBody and verify the signedPayload JWS to get information.

I have verified the signedTransactionInfo via jwt.verify. However, the result does not contain the appAccountToken property.

Are there any step(s) that I need to do in order to fetch appAccountToken?