On an approach where there is a DA based on the barometric altimeter rather than a DH based on the radio altimeter, the ADI DH selector being wound down to -20 means we won't get the flashing DH display on the ADI going off when it doesn't correspond to the DA bug on the altimeter. I guess that could lead to confusion i.e. thinking you're at DA when you're not.
With an autoland, where you have a DH based on the radalt, obviously the ADI DH flashing means you're at the DH for the approach, so it's not a problem.