Trying t help... Just to confirm two things.... You are trying to Mfa with the TC on, separately than without? Also, need to confirm setting that keeps the individual lens mfa's, and not using a global setting. My 1.4iii on a lens is 'non-detectible' when m shooting or looking at the results, just sharp.
Old news Guy, I was not MFAing this camera to fix a problem. I don't intend to go anywhere near that again any time soon. The answer was clear... so I got a different camera.
Guy I've had more 7DII's than any one guy should ever see in a life time. I have moved on.