Thanks, I’m going to search for some plugin (any recommendation?) and I will search for the android java code in search of the bug.
Meanwhile the only reasonable working solution I’ve found is to use preferences storage to save the product bought status.
In case a new instalation on a new device, there is no way to know so only when the user tryes to buy the product again, the bought status is detected automatically and stored in preferences… it’s not perfect and can annoy some user to have to click again in buy button to “enable” their previously paid purchase.