Defeated enemies remaining on the battlefield

During my Grand Arena I encountered a bug where defeated enemies would remain on screen. This was in a Starkiller vs Rey with Ben Solo. It ended in defeat as I couldn’t kill Rey, even though Ben was defeated and I hit Rey with enough to defeat her she remained alive. 

Am I missing something with Rey and she could have survived over 135k damage while in the red? Or has the bug meant she was unkillable as the game thought Ben Solo was still alive?