Cannot continue void flood, because Thrax enemies stay in an invincible state

[(XBOX)Kuhl MC]

Happened after the second round of destillers. Thrax enemy appeared, I shoot him down, switch to operator to finish him, but he just kneels down in a red glow with the enemy marker on top of him but no health bar. Had to leave the mission, because nothing happened anymore, except for the void saturation to reach 100%.