Hello, the checkout_completed event doesn’t work with third party checkouts which is understandable but I think it would be helpful if there was some sort of warning on the page for that because I’ve learned that a surprising amount of users use some sort of third party payment provider. If not, all good 
Hello Beckett,
Some events might not be available when going through third party payment processors, but checkout_completed should work as it is published on the Thank You page, owned by Shopify.
Could you provide more details or reach to support with specifics?