I was running a mission with a friend to try and get myself my second Kuva Lich. Said friend has an active Kuva Lich. When my Kuva Larvaling spawns, said friend is able to down it and perform a parazon finisher. He stated that it reacted like a typical Kuva Larvaling to Kuva Lich scene. However, the Larvaling is still there because it was intended for me to kill. When I try to kill it and perform the finisher, it never lets me finish it. It gets in that "finisher" state and will let me try multiple times in a row. However, the Larvaling simply got back up and returned to shooting me.
Needless to say, I had to run the mission again to get a Kuva Lich. However, the bug appears to be based on unintended actions with multiple players where the "finisher" is performed by someone with an active Kuva Lich.