At first, I thought perhaps my 160 had a problem with its back focus. At close proximity to a subject, at full zoom (manual focus), focus is lost when zooming out.

As explained by others below, this is documented and actually because at close range, the lens is in Macro mode, and at full (or close to full) zoom, needs to adjust to allow for such a tight shot at close proximity. You can see it in this video.

I'm happy to know there is nothing wrong with the camera -- it was simply alarming at first.

Camera was in manual focus, OIS OFF. I set focus on the pen jar, and almost immediately upon zoom out, focus racks away from subject, eventually returning when the zoom reached 37.7x. Same with zooming back in - focus drifts at 37.7x, peaks at 48.3x and then returns to where I set it at 77x. This is the transition period for the lens.

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