Arranged marriage in a nutshell, both prince and Euphyllia were made to become who they were with great expectations not and elto be close to each other.
So when a person who actually gave interest came by, they lost it. What a surprise.
Also, not confronting the matter directly instead basing the opinion on rumours of people that can gain power over animosity between the nobility, it's a massive flaw for someone who also grew with a burden of someone that too has a position to inherit it. Terrible parenting there...