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Honestly I have not worked out 100% how it works but here is what I have seen so far.
Engagements tend to work more if you perform them closer to the minor's coming of age. So the later you perform them the better but always before coming of age (you can do it otherwise). If two minors are involved again try to perform the engagement as late as possible. If the AIs controlled character becomes of age before your they will get married regardless however if you have performed the engagement late enough you will receive a pop up to ask you to confirm it once your character comes of age and if you do AI will break their marriage and proceed to fulfil the engagement.

So yes I don't think it is working as intended.