A/B Testing | Subject Lines (Sample)

Hi Marketo Community, 

I'm currently setting up A/B test for one of our blast email campaigns, where I will be testing two challenger subject lines both marked and weighted at 50%.

However, when I send myself a sample A/B test, I am receiving a number of emails some with the content looking as it should with imagery whilst others coming through are just plain text. 

Has anyone else experienced this behaviour when running a sample test?