When Minthara comes to you in the camp for the first time, she also feels calmness and peace in her head for the first time since long. You then have the option to tell her it is the artefact that shields her as well. I didn't choose that option but preferred to say nothing about it. So I don't know what will happen if you reveal it there to her. She's still an absolute commander at that point.
So in addition to the Emperor's constant attention there also seems to be a "radius of protection" inherent to the artefact.
It's all quite messy to say the least.