The lens is not parfocal in either direction. When you zoom, focus shifts.

What makes it seem one-sided is depth of field. At 18mm and f/8, depth of field is much deeper, so a focus error caused by zooming may still fall within acceptable sharpness. At 105mm and f/8, depth of field is much shallower, so the same focus shift becomes obvious.

So if you focus at 105mm and zoom out, the image can still look sharp because the wider focal length masks the error. If you focus at 18mm and zoom in, the longer focal length reveals the error.

That also explains why a crop from the 18mm shot can look sharper than a 105mm shot taken after zooming without refocusing: the 18mm image was correctly focused at 18mm, while the 105mm image was no longer in focus after the zoom.

In practice, with a varifocal lens, refocus after zooming. If you want the most critical focus, checking focus at the longer focal length is still useful because shallow depth of field makes errors easier to see.