A couple of things I can think of:

Wrong power steering fluid - use Volvo specific or the equivalent, incorrect steering fluid will destroy the seals/ o-rings inside the rack and pinion and make it stiff when making gross turns, but fine for minor turn adjustments - I have that issue and will be replacing the rack, pump and all fluid lines. There is a company on the web that makes a seal kit for the rack.

Or an air pocket in the rack, there is a procedure for flushing out air pockets when installing a new rack and pinion - check You tube.