Why is conversion rate to paying customer dropping in the last month?

Paying customers are active subscribers with a payment event. The first payment event happens on the merchant’s Shopify billing date. This may happen over a month after the merchant subscribed to your app. Recent conversions to paying customer data are delayed. 

Unless you’re measuring changes in your post-subscribe part of the funnel, you can use the subscribe/start trial conversion rate trend as a proxy for the overall conversion rate trend for the last month.