Oh FFS. I've seen the wider clip of the whole balcony. Everyone's standing and chatting, the anthem begins and they abruptly stop and turn and face the front.
Harry and Meghan are talking to the people beside them. Harry seems to be the first one in their little group to realise the anthem is about to start, so warns Meghan who looks like she thinks the next bit is going back inside. He looks a bit tense, I'm guessing cos he knows the press will go to town on her, yet again, if she gets it wrong.
The anthem starts, she cops what's going on and turns around. No drama.