Enemy adc would not die

I believe there is a possible bug, i just played lulu support with 9 marks of hybrid pen, and there was a jhin that was well below 100 health prob 50 or around there. he had no mr just his base mr, I hit jhin with lulus q got the slow and auto him his health went up instead of down. how is that even possible with hybrid pens and the jhin had no mr. his support didnt give him any sheilds the jhin did not have heal because he used it like an min or 2 earlier. How did the jhin live that?