Hard to tell through Luna's PoV, but I think Platinum might be waiting for the fourth shoe to drop. Whether she's at fault for Nebula's exile or not, she may feel something like survivor's guilt, especially now that she knows Nebula is dead. Also, she just learned that she and her family have been contributing to the Windego problem, possibly for generations.

There's still the question of whether she feels remorse or merely regret. From a Doylist perspective she's a stand in for present day Celestia, so I'm inclined to believe it's remorse.

I wonder if Sapphire somehow learned of Nebula's heritage, and that's the real reason she's terrified of Luna; if and when Luna's ancestry is revealed, ponies will find out that Sapphire bullied royalty as a foal. Regardless, I'm looking forward to seeing what happens regarding Sapphire. My guesses are that she's either going to try something incredibly stupid (like trying to kill Luna to silence her), or she's going to be made a political scapegoat so Platinum can claim the worst excesses of Unicornia's military were because of bad actors like Sapphire. Luna would hate that, but Celestia would probably take it if it meant peace between the tribes.

11804992
Ah, that's probably what happened. Thanks : >

11805713
What I will say, is there has already been a major hint why Sapphire is scared shitless of Luna.

11805912
That's fair.

11806097
Red tags would get hidden.