Admissions committees are familiar with the dynamics in various industries, and are no doubt fairly comfortable with job movement in media/advertising. The biggest concern would be hopping around to various jobs and completely changing direction, so that you are always starting over with an entry level position. In this case the changes make complete sense and contribute to your overall growth. I would absolutely go with the new job, and not worry too much about explaining the movement. Perhaps a line or two in an optional essay - or you could let your recommenders speak for you and explain it.
Hope this was of help.
Conrad and the Stacy Blackman
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