PMDG 777 KSFO 28L Threshhold Problem

In the PMDG 777 when crossing the threshold on runway 28L at KSFO the aircraft crashes into something that is invisible when the approach is slightly low or shallow. After hearing the "thump" of hitting something I immediately pause the sim and go outside and see the aircraft is about 30 to 50 feet above the runway. I unpause and watch from the outside to see the nose drop to the runway and the nose gear collapse. It's broken. Baro/alt is about fifty feet. Radio/alt is at -2 feet after touchdown. Can the radio/alt ever legitimately be negative? Sometimes a cloud of dust is kicked up from the invisible obstruction. I see nothing wrong with the terrain image and nothing wrong with the runway image. Can the radio/alt be set to a higher value? Or are the values given by the radio/alt being misread by the KPMG model? Or what? Thanks in advance.