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?
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?
Make sure you don't have any dynamic content on there, you could have a hidden segment with a different subject line