Assuming that you are not getting focus and then moving the camera slightly to miss it again, then
I think that the lens is focusing slightly in front of the subject. It appears a fairly constant amount at
the distance shown, and you notice it more as you zoom in - not that it gets worse as you zoom, just
that you see it better.
You have used f/2.8 for all these shots and that's OK. It might reduce with smaller apertures, but to have
it at all is still a pain.
Do a few more tests, and if you can, do it with a ruler perched at 45°. Focus on a particular measurement,
say 20cm, and see where the focus actually lands.
If you determine that it is front-focusing, then **hope** that your camera has micro-AF adjustment
(which I could not see in the manual
)
Alternatively, get accurate results with manual focus.
PS: It's not unusual to have an AF mismatch. I had it on a lens and fixed it with the micro-AF adjustments.
The camera remembers it for that lens. -- But again, do more tests.