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All lenses focuses wide open, only when the shutter is fully pressed, it stops down to the aperture you select and takes a shot.
Some non-parfocal zoom lenses shift focus as you zoom in or out but prime lens should not shift when stopped down.
Correct me if I am wrong.
Normally yes, but the well documented spherical aberration in the many superfast aperture lenses causes the focal plan to actually change when the lens is stopped down. Since focusing is done wide open, not stopped down, this can cause an issue.
The 50/1.0 L has a floating element which corrects for this, the 50/1.2L does not. Hence, it can suffer from minor focus shift under a very limited set of circumstances. Apparently it wasn't bad enough for you to notice it in your testing, yet many consider that "feature" to be a detriment to the lens.