Flag on Browns' first touchdown

So I watched the Bengals-Browns game with a party but the sound was muted. I saw the refs threw a flag on the Browns that they ultimately picked up. It looked to be an eligibility issue with the coach and the player brushing the jersey number. We were confused because we didn't think you could have an illegal man downfield on a run block, goalline play. Can someone explain the issue and why the flag was ultimately picked up?