Rek'Sai and J4 interaction bug

While I was ganking the enemy J4 as Rek'Sai, my friend and I noticed a really big bug. The J4 went into an unwarded bush and threw his flag down. I used my tunnel to get to him and knocked him up (Without vision) while at the same time we heard him pull himself to the flag. During that time and the next 4 or 5 seconds, J4 was completely invisible while still hitting us outside the bush. After he killed my friend, he became visible and was running towards the middle bush.

I don't know how to simulate this glitch again, but it gave him a double kill by allowing him to deal damage with Auto Attacks while we couldn't see him or target him.