I took about 50 shots of my daughter sitting at the kitchen table and everyone is front focused to some degree, I never hit one perfect. I inadvertently shot the first 30 shots or so in Servo mode, I was hoping that was part of my problem, but after switching to one shot I get the same results.
Al servo isn't the problem like you said.
Have you MA yours? Mine need +8...
I tested mine more yesterday and I can definitely see it bf (meaning my +8 is too much, 0 looks much better) at longer distance whereas at closer distance (99% of my usage) +8 seems to do the trick.
I'll just have to accept that shooting wide open all the time means I will always miss a few shots, that is I can't expect to hit perfect focus everytime, especially I use it on my 2 yo daughter which never stands still.
So far I am happy with it.
Here's a quick example, @ f1.6.
Bubble Fun by Kinson C | Photography, on Flickr