First, I'm an old fart and have never flown a 737 or any
modern airliner. Primarily I used the RadAlt to verify the reading of the pressure altimeter(s) as I dropped down through the maximum altitude in its range. My experience with RadAlts has been that they occasionally drop out and come back, depending on several factors including terrain. A RadAlt dropping out and coming back would hardly be worth noting, except that in this type of plane where it is an integral part of the equipment used automated precision approaches. Since the problem here seems to be a discrepancy not recognized by the RadAlt's internal logic, it may well not have even been noticed at all on previous filghts. If they didn't notice it, they couldn't very well write it up. The previous discrepancies were only uncovered by reading the flight recorder.