When a new user creates a subscription their roles are not being added after the webhook fires using the latest tested Stripe library (2016-07-06). It seems the problem is happening around line 517 of the stripe_subscription.module. The line $customer = stripe_customer_from_webhook($event); is returning false instead of getting the customer details.
I have tried changing the library version being used but am having difficulty setting different library versions and have raised an issue ( Library Problems → ) for that as well.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Because this patch fixes the stripe_customer module. and that's part of stripe, I'm going to move it to the other queue.
It looks like this fix was already included over there, so I'm switching this issue back here, but marking as support/documentation so that it will hopefully help the next person: the solution is to update the stripe module, and this problem should be resolved.