Never exchanging hostages again

I am playing a Han Chinese empire on the east side of the map. Many of my neighbours are tribal so I get invaded occasionally. I exchanged hostages with the khan of Khakassia, ideally to prevent him from attacking me.

All was well until Buryatia declared war on me to conquer some county in bumfuck nowhere. As I was raising my armies, Khakassia joined the war on Buryatia's side and immediately executed my hostage.

This game is dumb sometimes.