If it only happens with that one Sigma lens, you are right. Canon is not going to fix something caused by the lens. Sigma has reverse engineered Canon protocols, and tells the Camera that it is a certain Canon lens, and the camera responds accordingly. There is a table of Canon lenses and camera corrections for them internal to the camera. It its making the wrong correction for the lens, then sigma is at fault, and they are the only ones who can fix it.
If you have the lens correction feature enabled in your camera, turn it off and see what happens.
Yes I do have lens correction enabled, I'll turn it off and see whats up.