Same complaint here. I was about to implement (incorrect and dodgy) functionality to extrapolate the billing period end by adding 30 days because it seemed like currentPeriodEnd was being set to the end of the last billing period rather than the end of the current billing period, and I assumed that was normal behaviour.
This lack of functionality is really concerning, and nearly caused us to make a major implementation mistake that would have broken our user subscriptions once the Shopify billing was pushed to production.