Your post inspired me to test my lens via focal. Here are the results
Sigma 50 ART:
Canon 85 1.2II:
Note that although the Sigma 50 seems to lose some resolution on the right AF points, its still well within acceptable range. (.5+ is considered acceptable). The 85 1.2 performed better, but not significantly so. It should be noted that the room I tested this in was lit via a window to the right of the frame, which could have affected the AF performance of both lenses.
24-70 2.8II, which has a particularly reliable AF system, is expectedly pretty consistent across the frame. General sharpness across the frame likely comes into play here too. We know that the 24-70 is tack sharp across the frame, while both the 85L and Sigma 50 lose resolution in the mid-frame where the outer AF points lie.
All in all, in my use the Sigma 50 ART focuses rather reliably across the frame. You might want to send yours back in.
Real world shot, right corner AF point (worst point according to my test):IMAGE LINK: https://flic.kr/p/nQgEEx DZ2A2510
by Carlo Alcala
, on Flickr