You know, my avant has this issue, or at least a mild version of it.
The steering feels "tight" compared to my saloon. Theres plenty of assistance, but its as though something is applying friction to the steering wheel as it turns. As a result, you can dial in a small steering angle either direction and it'll just stay there rather than returning to centre. Once you turn the wheel more than maybe 1/8th of a turn, there is plenty "return to centre" force applied to the wheel, but as the force drops away as it nears centre, it seems the tightness is enough to overcome the self centering.
Its actually been annoying me quite a bit recently, but i've always noticed the steering in the avant seems heavier/tighter than the saloon.
What i really need to do is disconnect the column from the rack, and see if the column turns freely. If so, then i guess it must be something internal in the rack itself.