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Sorry guys I know it looks like I'm spamming but the asterisk's are there so this post doesn't get lost in the deluge of responses. Like you I used to think it was a bug BUT IT ISN'T. This is by design.
I'm probably one of RIOT's biggest detractors on this board, I speak my mind all the time I am no RIOT fanboy but the reason they put this aggro change in was GENIUS!!! Of course I don't want to say why because the reason for this aggro change will put those who know how to use it at a disadvantage but feel free to check my MMR over the past 3 weeks. RIOT games have done something marvellous.....FOR ONCE!
Massive props to this small, subtle but significant change. Yes, it's annoying to know your minion is going to change aggro as you go to last hit it. But there's a reason RIOT changed the code that's existed for 5 odd seasons.
This is a rare occasion where I say well done RIOT. This is a good thing you've done.