From your description it sounds like your login when they start your app is mandatory. Apple forbids this. They insist that the login be optional, but it must still be available to everyone who wants to use it (when using non-renewing subscriptions).
What I did on my app was, next to where I show the user's purchased subscription level, I have a button that says Save your account...
. And when they click it it describes to them how, if they create an e-mail and password than they can share their subscription across all their devices and they can restore their account if their iPhone gets lost or stolen. You want to incentivize the user to do this because a better experience for them means a better experience for you. You could even offer them a perk, like 2-weeks free if they create an account. And since you're using non-renewing subscriptions, you have the power to do this since you're calculating your own expiration date, instead of letting the app store do it for you.
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