They only target one character.

They always attack only the 3rd character – the cleric. They don't start attacking others until he's defeated. I'm just shocked! What could be causing this?

This problem started after the Emerald Isle, if I remember correctly.

P.S.
After playing around a bit, I noticed a strange thing – in the Harmonydale castle, they attack everyone, but in the open air, they only attack one character.