B&W changed the install instructions for the turnover ball at some point.
The instructions that came with mine state you can install over a hardshell bedliner or mat.
But the latest instructions available for download from B&W's website states the bedliner or mat "should be removed, or must be cut" to allow direct contact with the bed, though a spray-in liner is acceptable.
See for yourselves in Step 3 here:
LinkI cut mine, I figure there was a reason for the change.
But I'm sure thousands of setups have been installed using the older instructions without incident.