I don't know why - but since a couple of months it usually takes about 15-60 minutes for Paypal IPN to send the IPN notice of the succeeded subscription payment.
Now if the user receives the email about the payment instantly (the paypal emails are usually very quick) - and cancels his subscription before the IPN notice of the payment arrives - then his account will be wrongly demoted.
This is pretty annoying and happens to me lately quite often.
Now if there is still any development on s2 - then please add some sort of timer to wait for IPN messages for 6 hours after a subscription is canceled on paypal.
This bug is not new - but now much more common than a few years ago because paypal IPN became so sluggish.