I see two things causing trouble when it comes to Andruil :
First obviously ,she went to the abyss , and went crazy.Mythal fought her , and stole memories from her.
Maybe she didn't trust Andruil , maybe she thought she was too corrupted and would go back despite common sense.
Anyway it doesn't sound like Mythal trusted Andruil.
Second : Andruil promoted Ghilan 'nain to Godhood.
It had some ripple effect if we believe some codex in the temple of Mythal.
Ghilan 'nain went to see some random elf , and told him he could shapeshift into a form reserved for the Gods.
It didn't end well , it was forbidden.
So the elf asked for Mythal mercy , she refuses to get involved , and let Elgar'nan deal with the man.
(the text ends up talking about a angry unknown mass , I assume the "judgement" was received poorly)
I'm thinking at one point the elves started to question their "Gods" divinity, Solas refuse to see them as "Gods" afterall.
Sometimes I wonder if Solas was that elf who shapeshift , since he's supposed to be the God of rebellion.(and there's also some old tales of a wolf flying in the sky , and it seems the Divine form was the dragon)
He talks about being betrayed and not trusting your friends .
It's possible it was all a plot to make Mythal looks bad , and turn the people against her.
And then she was murdered and without her influence , the remaining Gods only got worst.