There was a leakage path in the radio altimeter antenna system that caused the -8 feet indication. At some point on the approach, valid ground return would have been enough to overcome the signal strength of the leakage path. That's why erroneous radio altimeter indications don't show up at the gate, and require more than "black box swapper" skill to troubleshoot.
Swapping RAs left vs right would have led to the news that it was not black box trouble, but rather in the left cable or antenna. I thought all techs employed this old, old method of sorting things out.