In short: It's not possible to start with heavy attack after Mr.Sinister's Sp3 animation ends. What happens now is that he starts with a medium attack.
Detail: So, in order to extend the duration of sp3 power leach I just had to parry, hit sp3, and hold right-hand side of the screen while the animation is playing. Once the animation ends, Sinister holds heavy and extends the stun and power leach. This playing style was also encouraged in the champion's developer's node.
Example game play before update:
https://www.youtube.com/watch?v=97k8im6-d60What happening now, is that instead of starting with heavy, sinister just throws a medium attack which does not serve any purpose.
After update gameplay:
https://www.youtube.com/watch?v=hr1Gx1jgLRoI don't know if it's a bug, or it's an intended change. Any clarification is greatly appreciated.
I use LG G6
Android 8.0.0
happens on both Wifi and data