that wasn't a mystery reaction, she was just shocked to see Momo still indecisive this late into the story
In the langage of comics/cartoon, that specific element symbolize a trigger in the mind of the character, something that will prepare a change of mind.
This was what followed that interaction
On your page, Momo and Yamato are arguing about surrendering. On my image on the other hand, Momo is wondering about closing the border because he wants to protect the country and ask Yamato for advice.
Those two themes are different. The subject of the first conversation has nothing to do with the other.
Yamato definitely snapped while hearing Momo's questionning. This.. was not brought up again.. but it should! If that's not done, that "snap" would be meaningless.
My conclusion is that on that specific moment, something changed deeply in Yamato's reasonning. Preparing possibly a change of desire in the future.