A few thoughts, which may be of use.
For Ivan's image, I think Bart could well be correct. It looks as if the leaves above the bird, are more in focus, and I think they may be behind the bird. afaik, the af works on a high contrast line on the object. The high contrast line is where? the edge of the stick? so does it focus on the stick end, or the bird? AF can be confused quite easily. So, perhaps the camera focusses on the stick edge, then possibly moves some distance, detecting bird feather movement and keeps going since it gets the higher contrast area, (mentioned by Bart) could be one rough explanation. If Ivan has only a few oof issues, there may be a consistency within them, and the cause may be determined by careful testing. It may be that in certain conditions, the behaviour is slightly different than may be expected.
For Paul's image, and I guess many are similarly blurred, the hair looks as if it may be more in focus, but that maybe the nature of hair. I don't know the depth of field for that lens/distance, but I guess if focussed properly, everything would be sharp (except the background). Is the lip area high enough contrast, is curved lines a problem? It may be it is sort of retaining the focus from where it was yesterday, so to speak, it's caught out, looking for sharper movement/contrast and then Paul presses the button. What to do? say 'I'm not ready, no image yet', or 'you know best, is this good enough? I think the camera and lens may need calibrating together, since the problem seems to be consistent. The way this stuff works, the speed at which it works, is quite amazing. To get the speed, there has to be tolerances in the components. Sometimes these tolerances add up, sometimes they subtract. Trying the lens on another camera proves the lens on that camera, it does not prove this lens/camera combination. (If the lens used to work fine on the other camera, but now it doesn't, that does not prove that the lens is faulty, either.) It may be worth trying with is on and then off - more user testing, I guess. I can see people walking around, carrying trays of spice jars ;-)
Best wishes,
Ray