Okay, so the game didn't randomly get rid of the alliance, I know exactly why it ceased to exist.
I'm the king of Sicily, and making snugglebuddies with the Byzantines to avoid being conquered by them. I marry my heir to a member of the ruling dynasty and everything's nice; I've got a nice strong ally, and I needn't fear them deciding that I'm tasty.
Then I win a crusade for Jerusalem. I tell the game to distribute my titles to save me the effort, and it promptly gives my first born son, the guy with the realm-saving marriage, a bishopric. The marriage vanishes, and the alliance goes with it. I, of course, can't establish another alliance because of 'political concerns'.
Surely this cannot be working as intended. The arrange courtier marriages decision makes a point of avoiding your dynasty members, after all.
I'm the king of Sicily, and making snugglebuddies with the Byzantines to avoid being conquered by them. I marry my heir to a member of the ruling dynasty and everything's nice; I've got a nice strong ally, and I needn't fear them deciding that I'm tasty.
Then I win a crusade for Jerusalem. I tell the game to distribute my titles to save me the effort, and it promptly gives my first born son, the guy with the realm-saving marriage, a bishopric. The marriage vanishes, and the alliance goes with it. I, of course, can't establish another alliance because of 'political concerns'.
Surely this cannot be working as intended. The arrange courtier marriages decision makes a point of avoiding your dynasty members, after all.