Having taken apart a few (non e46) racks, there IS at least one way to get a rack wrong.
There's a preload spring and slider (or other arrangement to provide a controlled pressure) to keep the pinion
and rack tight. If that's way wrong, I could see it making a mess of handling, as the rack will deflect at the
driver's side, change the effective toe, and off you go.
I'd trust your expert- try ZF to see if they'll give you help, and then swap the rack for the used one.
It's easy to inspect the rack itself (pull the driver's boot, turn full left) and if it looks ok, try it.
I hadn't heard of this either, but it's certainly possible.
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