When the user clear data and storage on his device, the setting isSubscribe becomes NULL? if yes, do not initialize it by FALSE, but NULL instead. Check restorePurchases() anytime you have this variable NULL. (Having this variable NULL means either the user has launch the app for the first time or has deleted his local storage. At this moment, check restorePurchases() and set the local setting to TRUE/FALSE).

NB/ I'm not sure that restorePurchases() check the status of the subscription on the google server or locally because in many researches I have found that google recommend checking this status on the server side using Developer notifications for in-app purchases)