Oh. Ok. Excellent. At least now I know why I didn't get many keepers shooting near MFD with my 85 1.8 at 2.8 when using the outer focus points. Not very happy.
You may have an issue, but I'd do a controlled test first and make sure it's repeatable. Don't want to start a panic among 5D3 owners with this thread and get everyone paranoid when only a smattering of models are affected. Let us know.