Aside from the AI angle, there’s actually another way this weird bug can manifest.

If you’re following best practices and sending plaintext alternatives with your HTML email, then some mail clients will use the plaintext for the summary snippet and render the HTML when you open the email. So if a developer copies the success templates to the failure templates but only updates the HTML and forgets to update the plaintext alternative, then you will see this exact behaviour. It’s also pretty tricky to catch when manually testing because not all mail clients act this way.