I was left thinking why 3PO and R2 got on a Corvette that was docked in a capital ship that was heading into battle. I also wondered how quickly Evazan left Jedha to get to Tatooine. Must have been on his way to his ship?
The Tantive IV was on a mission to Tatooine before the Rebels committed to Scarif. Plans changed. Wouldn't be surprised if that Mon Calamari admiral jumped the fleet to Scarif before it had a chance to undock.
We also don't know for certain it was docked at the start of the battle: it could have docked during the battle when it was clear the flagship wasn't getting out of there.