<non pilot here, but aerospace student in human factors>
If the aircraft indeed went into retard mode, while being way above 28 ft RA, I'm very concerned. While it is strange that the pilots did not notice the speed decay for such a long time (possibly relying on automation to maintain it), it should technically not have gone into retard mode at that altitude. If two radalts are available and they disagree violently, this information is not suitable for deciding to switch to retard mode.
It could show something that many believe: Automation cannot anticipate for all cases, and should keep the human in the loop for exceptions like these and alert the human of conditions that are obviously violating the flight regime envelope/constraints.
Anyway, there's surely much more to this incident, I will follow it carefully.