They were in the midst of a fight (sort of) and Istredd was expecting a push back, getting ready for it. The thing is, no matter how much they argue the point, when it actually happens, instinct will win out. He can promise here and still feel that gut fear when it happens and find him. He couldn't save Yennefer when she was attacked, he couldn't force everyone to do their cards.

Whether it makes sense or not, whether he wants it or not, the two of them are connected in his mind. Lucifer maybe planted it there himself by saying they were alike. She is his biggest blindspot and he didn't expect another one would happen. It's not good for him either, he has plans. But he is telling the truth. This is where he's at. He hopes it never comes to pass though.

What he absolutely did not expect was Lucifer to just stop. Freeze. It didn't make sense so it threw off his anger. At first he is concerned someone is attacking his mind, but he would know that. His eyebrows furrow in confusion and he cautiously takes a few steps toward him, closing some of the space Lucifer made. "Lucifer? Hello?"