Well known problem - there was a TSB for this and the mirror was redesigned in 2008.
The mirror position sensor wears out, so the computer can't put the mirror back into the correct position after selecting reverse.
The lexus fix is to replace the mirror. I reckon it could be done by taking apart the motor unit, cleaning it and using a dedicated potentiometer cleaner to clean the position sensors. I've been meaning to do this to mine, but I just turned off the auto-dip function, and haven't got the time or motivation to try it.