Two related symptoms were reported. Autofocus would fail to achieve focus when shooting at a closed aperture, and focus would shift when the aperture was changed after having been set manually.
Because both depend on a particular sequence of operations, the fault is easy to miss. Someone shooting mostly wide open, or focusing before stopping down, may never encounter it, while someone doing landscape work at f/11 might consider the lens defective from the first day. Two owners of affected copies could reasonably give completely opposite accounts of the same lens.
The free repair programme has closed, so an unrepaired copy on the used market now carries the cost of correction.