Stripe coupon applied after 2-3 payments

We have a subscriber who subscribed, made their first payment and then somehow applied a coupon to their next set of payments. So they went from paying full price to paying 20% off. And I can't understand how they did that.
And I need to change it back without canceling their subscription. According to MS2P, all her payments are full price. But according to Stripe, she somehow applied a 20% off coupon, which we gave her. And the coupon does exist – I created it, but I didn't know it could be applied to MS2P purchases.