@airbusa330
BOAC, you haven't missed anything. With a radio altimeter inoperative the autopilot will disconnect 2 sec after LOC/GS capture.
Are you are sure you aren't getting mixed up with autothrottle disengagement 2 seconds after touchdown?
BOAC, I am not sure if you are B737 or not, if you are, have you found anything in the manuals regarding the A/T logic of switching from left RA to right RA with a failure?