Possibly the beam is bent meaning you have to correct it to keep in a straight line. Measure from middle of rear wheels to the edge of the door (or other reference point) and compare to each side.
I would be okay with 5mm or so myself but maybe someone with more knowledge in this area can correct me.
Note that the rear suspension angles can not be adjusted by tracking. If the back is out then it will be hopeless trying to bodge the fron end to suit.
Hope it turns out to be something daft like a beam mounting bush or similar mate.
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