Unfortunately this act will later result in angering the other ducal houses. After the saintess regalia were determined to be ordinary ornaments, the king distributed them across the ducal households, which they took as an award of prestige and honour for serving the royals.
By returning the tiara, the dad has unknowingly set a precedent for the royals to demand the regalia back from the other houses, which is akin to withdrawing said award. And noble pride being what it is...