GAC. It is impossible to kill any of the team of SLKR. 1 unit of health remains and they become invulnerable.
Ideas
Dark Rey prevents First Order allies from being defeated until all enemies have taken a turn of are defeated. Did everyone on your team get a turn before they killed all of them? That datacron will give Rey a bunch of bonus turns.
This is the answer.
TLDR version - Bug? No. Read kits? Yes.