This would appear to be a bug in their API. Drafts moved to using v2 of their REST API, because they are discontinuing v1…but this doesn’t seem right. I don’t have time to dig further right now, but if you force Drafts to dropback to the v1 API, it works - and the docs between APIs do not suggest there should be and difference.
To do that, add to.apiVersion = "1" after you create the to object, but before you make any calls.