That happened to me once only it was a normal battle. I defeated a nirini but he somehow revived with 1 health. It even did the thing where it shows the nirini dying. Please note I do not remember what exact type of nirini it was or where it happened. At first i didn't know it was a big deal. My advice would be to ignore it and move on. Mostly because I WAS able to finish it off .
I have had this happen as well on multiple enemies in various areas. The first time this happened, I thought it was gone then it attacked me the next round. I noticed that it will happen more often if I defeat them with exact number of their health points and take them to 0 instead of below 0. I just hit them again and move on.