Rengar De-aggro when target walks away from rengar

ive noticed that when i combo with ult if i dont constantly click the target rengar will just completely stop attacking or switch targets when in a minion wave. Its super frustrating when you go to get a pick and after the initial combo rengar just randomly decides to stop attacking. Since i realize that most will probably think i just suck and am miss clicking, heres a few clips of me showing it off.

Clip #1 https://plays.tv/video/5a7a7e3ee55f8819f4/-buggy

Clip #2 https://plays.tv/video/5a7a7f8de09dad87a8/-hmmm

Clip # 3 (back to back) http://plays.tv/video/5a7a85d884d32ef361/-2more

id say maybe 1/5 of the time for me and it only happens if the target is walking away from rengar.