For instance on Android when a subscription is auto-renewed does the payload purchasetime get updated or remain the time of the original purchase?
Subscriptions are one area of the the product that we are actively working to improve.
Right now, one of the shortcomings in our system is that we only contact the store during initialization (to check for product availability) and during a purchase (to complete a purchase and get the receipt).
I understand that it would require some extra effort, but you might be able use the Google Play API to access the subscription information:
Would be helpful, in the mean time I am using Google’s cloud functions to update my online database when a subscription is autorenewed or cancelled, within Unity I am just checking the database instead of using the IAP receipts.