Would you not expect A/T to disarm automatically with a big fat warning when RA provides "illogical" data as in this case? -8 would be a value which only a defective RA can deliver as the lowest on ground is -2.
Suppost at 1950 there would have been a warning about the defective RA and A/T would have been disabled with a clear warning (not gear down as RA is the issue), then crew would have been warned with correct situative information and would have prevented this from happening.
Systems that do not provide clear and precise feedback are only good for disasters waiting to happen.
Afaik Boeing need to fix this flaw.