Simple Commerce doesn’t register the subscription but does register the payment. Seems to be related to the bug at the link below but no one in thread has responded to my requests for assistance. One of the commenters suggests a 400 fix but doesn’t go into detail.
https://support.ellislab.com/bugs/detail/16607
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“responding with a 400 - rather than just ignoring the payment IPN for a subscription that doesn’t exist - corrects the problem for me. Paypal correctly re-sends the payment IPN.”