I've had this issue come up in both games I started. Once as Yohn Royce, the other as Rodrik Harlaw
In both instances a well-known storyline character asked to marry one of my daughters. As soon as she moved to their court, she moves straight back. The character would then re-propose and the loop would continue.
As said you can break it by rejecting the offer and marrying her off to someone else, or by instigating the wedding yourself.
Edit: both daughters were young and not widowed