I have changed my LCD mode from A to one of the others (*1 *2) and still it auto brightens and darkens. I have no problem changing it in the video modes through the exact same process. Am I missing a step here?
You can do this by pressing aperature -> display. This enables the " SHTR SPEED EFFECT OFF " and shows you a more accurate preview. You can also go to page 4 of the Custom menu and set CONSTANT PREVIEW to OFF.