I did a few calculation to find out what error would be necessary on the back adjustment to get the kind of error you seem to have. Let us say that you have a 100 mm lens, focus 2 m away and get a front focus of 2 cm. Using Descartes formula ( 1/p+1/p'=1/f ), I find that the back is 50 µm too far. That is about half the size of a human hair.
If you talk to a machinist, you will find that milling the contact surfaces between your camera and the back with a precision of 10 µm is an achievement and that temperature drift will probably mean your surfaces are not at the 10 µm position that is necessary. Maybe the camera AF electronics take temperature drift into account, I don't know.
Anyway, the message here is that AF accuracy is dependent on the four shining metal squares around the sensor on your back and on your camera are positioned with an accuracy of 10 µm or better. Obviously, any dirt on these pads will have some thickness, move your back away from where it should be and cause some front focus error. You don't need much dirt to be 10-20 µm thick, 10 µm is a tenth of a human hair size.
If everything is clean and you have mounted and dismounted your back a few times to insure that these 4 surfaces are correctly mated and you still get front focus, I would ask the back manufacturer, since you said that your camera has been adjusted recently. In the back, there are positioning screws to move the sensor a tiny bit forward. With the right equipment, this is a trivial adjustment.
