When I use addon, it tells me the loop is

But the simc results are

https://pastebin.com/HrkYWFR5

I think there is a problem with the condition pet.fiend.down, where simc means fiend already exists, and addon means fiend is gone!

I moved your snapshot to pastebin per the instructions (so it remains readable). There is some tuning for Insanity generation that I'll work on, that will get that prediction in alignment with simc more proactively, but keep in mind that the addon still reacts to actual Insanity generation and is going to update to recommend Void Eruption as soon as you actually have the Insanity for it it.

What makes you think pet.fiend.down means that your fiend is up?

Reading through the sims, it does look like you're right about fiend. Which is massively stupid. pet.X.up means the pet is active in every other context, so I'll have to make this backwards for Shadow Priests in particular.

I've submitted a ticket for simulationcraft to explore this behavior; I really think they should be using pet.fiend.up to reflect that the pet is active (similar to how you'd use buff.voidform.up to show that the buff is active).

In my working copy, I've modified the logic for Shadow Priests with the following result:

I'll post a new alpha or beta soon that includes the change.