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The ZE 100/2 has floating elements, so the focal length probably varies with focus distance. And when you put it on tubes, they might obstruct the light from the rear pupil of the lens. Or focus could actually be behind the front element, which would of course give vignetting and nothing in focus.
That sounds improbable to me, but maybe you're right. I have a Tokina 90/2.5 which also has a floating element but it works fine even on a fully extended bellows. The way to check would be to start with the full set of tubes and the lens focused at infinity and then see what happens to the focus distance when you turn the focus ring gradually towards the MFD.
By the way, I believe focal length always changes with the focus distance, except for lenses that don't have focus breathing (like cine lenses that are corrected for this). It's lenses with internal focusing (which this none of the Zeisses have) that have the strongest reduction in FL.