Why does Namharim, Born of Night never phase?

I don't know why but I consistently have issues with this harvest boss in particular being immune to actually going into its next phase. the other bosses get it sometimes too and im wondering if theres a specific trigger. It is so painful to try and farm harvets sometimes because they exist

I think its just damage. A bit like Drox, middle phase can seem like you are doing no damage sometimes.

Not certain though, I've only done these bosses a few times each. A few times it did seem like an endless summoning, think I might not of been doing enough damage.