Free Trial Not Automatically Turning to Paid

I have an app called WikiSleep. It is a free sleep app with a basic subscription base that activates after a free trial.

Some users who have been given promo codes are able to use the app in full, even after the promo has expired. This one, for example ended months ago but the app is wide open for him. Any ideas on why that might be happening?

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Free Trial Not Automatically Turning to Paid
 
 
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