Dispel evil and good vs. Myrkul

Hi I'm mad and can't find anyone else mad about this and need to know if my game is bugging or if it just doesn't work that way, and if it doesn't why?!

I'm playing custom after a failed honour mode to prepare for a redemption honour mode hopefully...

I'm playing my monk and Shadowheart and my partner is playing his sorcerer and Laezel, but most of this doesn't really matter.

My main gripe is that I cast dispel evil and good on Shadowheart, a spell I assumed might be useless but I saw it and was like okay this might actually be useful against Myrkul. I go to break the frightened condition on Aylin and it says that the person isn't being impacted by a celestial (I think). I mean she wasn't but dispel evil and good had the whole list INCLUDING undead and when you examine Myrkul is clearly labelled undead. Am I stupid? has anyone else tried this?

We lost the fight (I don't need tips I've seen them now in trying to find this, we'll figure it out) but I'm more upset the spell didn't work as advertised.