Using a reduced test case, I’m pretty sure my issue is a symptom of this same issue:
https://answers.netlify.com/t/send-response-before-stopping-function/5016/12
The function which I’m trying to run this code is async.
If I make a reduced test case, sending a simple email using Postmark in a function that is not async, it works.
Question is - why do async functions work as expected using netlify dev but not in production?