While I don’t believe in the Great Tyrell Conspiracy there’s something that gives me pause. Given Cersei’s letter to Jaime asking for help we know Dragonstone falls before Riverrun, yet when Miranda gives Sansa the news during the descent she mentions the Tully yielding but Dragonstone still holding for Stannis. Am I missing something?
Most likely, it’s a case of incomplete information – that news has reached the Vale about Riverrun but not yet about Dragonstone.