I got this three times in row with my current game, in which my heir is bethroted to a genius (below 16) and then given a baronny.
Ahh, I found a fix to this but I forget how I did it. IIRC in order to avoid this, you have to go click the individual arrange marriage command for the particular character in the diplomacy screen. I haven't played in a while, so forgive me if that's wrong.Yes! I've had this as well for my emperor and it really irks me. But furthermore I've also had heirs for whom I couldn't arrange anything, neither a betrothal nor a marriage. It really is in some cases rather gamebreaking.