Lich insta killed - no progression

[Keetru]

Was doing a lich defense on Casta-Ceres. My lich had spawned under the moving defense point, I was next to it called in my archgun, and around the same time as the archgun knock back hit it all of its health disappeared instantly and the lich had dialogue as if I had successfully did a parazon execute with a correct mod order. At the end however the lich is still on Ceres not progressed to another planet or onto the railjack phase.