Gmail Email Markup validates and passes SPF but fails to render

Question

I'm using the Parcel Delivery schema and have had success all the way up to the point of rendering correctly inside Google Inbox. I'm still at the point of testing so I'm sending email from [email protected] to [email protected] which should be enough to test the implementation.

1. The email content & schema passes the Email Markup Tester
2. The email content & schema works correctly when using the App Script Quickstart. The email arrives in my inbox and renders correctly
3. The email content & schema works correctly when using the App Engine Tool. The email arrives in my inbox and renders correctly
4. I've sent email where the 'to' and 'from' email addresses are the same
5. SPF and DKIM both pass when I inspect the email sent

Kind of stumped with little to go with to assist with debugging.

Worth noting that I use Mailgun to deliver email from my service.