During one of our A/B testing based on the subject line, Marketo used the name of the email program instead of one of the subject line defined in the program. I have checked the initial subject line within the email copy and it was not the program name. Is there a bug we need to be aware of?
Make sure you don't have any dynamic content on there, you could have a hidden segment with a different subject line
I personally have not had this happen, assuming your set up is correct. Could you post some screenshots of the campaign set up? Have you checked with support?