The Marketo parser will stop when it hits an emoji and clip the rest of the code. This one was weird, because I often hand off coded emails to internal teammates who then set them up in Marketo. They reported back an error that didn't make sense, as the emails were complete documents:
HTML error, '/body' tag is missing
I double checked the emails in Email on Acid, sent some tests via Customer.io, and then eventually got access to the Marketo items themselves. After pasting, I could see that it was clipping the contents of the email and was suspicous of the emoji. Removing them fixed things up, so back to images we go.