>> ...I wasn't expecting the camera to focus on the guy's face when the focus point was clearly on his abdomen. I was expecting the camera to focus on his abdomen, though, which it did not.
Regarding that image 701_1009...Now that I have more time to fully respond to this...
There are at least two of us here, who think the carpet is in sharpest focus at just about your subjects knees, or perhaps a few inches in front.
That is probably consistent with the 20 degree downward shooting angle previously suggested. And if we are correct that that point in the carpet is in focus, then the subject's abdomen, on which you focused, is also likely in focus.
Now, if the carpet immediately in front of the subject is in focus, why is the abdomen "out of focus"? If it's really in focus, as we believe, it must be a tiny touch of motion blur softening it up. And if something is moving, which? The subject or the carpet?
That is why I questioned the 1/80s shutter speed. You may not have moved when you shot that self portrait 5 years ago, but perhaps your subject did?
If you disagree with the above then please tell me where you think the carpet is in best focus. If this is an example of your camera "background focusing" then it must be in sharp focus somewhere well behind your subject. If you can find that sharp carpet then maybe you will have a convert.
One other important question. During the 7+ hour interval spanning just those images, you shot well over 400 frames. What percentage of the images have the problem you report?