I've got the same issue! I hope OS Sierra hasn't ruined my League career. Didn't have a single issue until I upgraded. I have 2 steel series mice. Also thought I had some sort of issue, so I switched my Kana V2 for my old Sensei whereby I can use the V3 engine, but it has the same issues. Checked settings and worked with in game league "Software mouse" settings, but on or off, the shit still jumps every once in a while. Fucking me up hardcore when I'm in team fight pushing a tower and it clicks me into their tower next to 5 enemies. NOT cool. Died so many times cus of the seemingly random mouse jumps.
Based on reading these comments I can conclude it seems like it is NOT a mouse issue. I will also try to delete and reinstall league, to see if something got corrupt, but someone here said they already tried that and had the same issue.
Aside from that, the only thing I can think of is 1) Some new setting in Sierra, or 2) League client has some sort of bug with Sierra that causes issues with mouse flicking.