What a clusterfuck of a situation.
Elizabeth asked the prince to deny engagement to anyone in the future if the adults tell him about their planned engagement. It's because she knew she was the one who he'll get engaged to. Instead of realizing Elizabeth knew about their engagement as a duke's daughter and didn't want to get engaged, he went on with the delusion that she secretly wanted to be with him via true love (despite Elizabeth not bothering to keep in contact with him for a long time).
Whether it's because of the game's plot having an effect on the world or not, I don't know.
Part of it is her fault since she didn't clarify she didn't want to get engaged AT ALL. The other part is that the prince isn't giving her any chance to reject him. As a nun and banished nobility, Elizabeth can't reject the prince's offers due to the way their society works. He's also a yandere so rejecting him will just trigger that side. The prince is either denying the hints Elizabeth has been giving like literally running away or just doesn't see it due to his delusion from childhood.
In a way, it's the perfect set-up for Leo since Leo does not care about societal norms and doesn't fear the physical threat of the prince ordering people to attack him. Can't wait for the next part. This gonna be good.