No chance he does not know she was REQUESTED from another country.
He should assume that that country is going to want her back, safe and sound.
He is acting like it is purely internal politics, but it is international politics at its worst, because calling her a fake is a direct insult to the honor and integrity of the nation that sent her to them.
My guess form his coldness is since she did not state her lineage he assumes she is a commoner and feels insulted at being engaged to a "lowly" commoner.
So basically, Prince Prickface went "The Saintess is fake, her powers are nothing."
Princess Saintess therefore went "Welp, looks like they don't need me to keep up this MASSIVE fucking barrier that's continiuously draining my power."
Chapter 1 in a nutshell
Arthur: You a fake saint hurrdurr you deceived us hurr durr (all according to plan)
Mira: Alright, adios. I ain't gonna put up with your BS.
"They don't need me anymore so I guess I won't be protecting them anymore"
Quite the punishment just for dumping her.
Then again, they didn't want her anymore and even tried to hurt her physically, then hurt her emotionally for real.
She's in no obligation to protect them anymore.