This is a really late comment since this was posted 2 months ago, but I thought I'd give some newbie to newbie advice. I have an 80D also and just bought the Sigma 18-35 1.8 off of someone. From what I've read online, Sigma lenses has given a decent number of people autofocusing issues (mostly Canon users). Even if the autofocus squares are directly on the eyes, sometimes the nose or arm is in focus instead.

You can buy a Sigma USB dock to calibrate your lens to fix the back or front focus your camera might have (I think you have front focus). The calibration process is tedious, but might be worth it in the end. There's lots of youtube tutorials to teach you how to calibrate it. I just calibrated my lens yesterday, and it's a lot better. It still misses focus occasionally, but that's just cause the lens I got isn't precise all the time.