Have you tried checking the EXIF of a raw file with some decent software, eg. exiftool to see what that says? Looking at the EXIF in one of your 300-800 files only shows what photoshop has written not what was there originally.
Does the same thing with my 150-600 OS C thinking its the Canon 100-400
I actually think it might be a bug in how Sigma reverse engineered the Canon mount and such..
IMAGE LINK: https://flic.kr/p/NKQjeV Fall Squirrel 5 by Trevor H, on Flickr
IMAGE LINK: https://flic.kr/p/MVta78 Fall Squirrel 6 by Trevor H, on Flickr