Is there a way to stop heir from randomly marrying?

Just 4 months away from having the the crown prince of Poland come of age. I have been saving my eldest daughter after all her male siblings died. Nevertheless, without hesitation they will marry some backwards noble from a opm minor in Germany who is 6th in line.

Why is this happening? I have given my daughter cabinet rights to ensure they don't leave the court. I have watched their location to make sure they don't end up in one of my fifedoms or other PU members. Somehow, they still manage to marry this clown. A betrothal button would be nice. That said, they free movement of the royals between titles is pretty annoying too.