The most important thing for auto exposure is to make sure that the histogram min and max cover the range of brightness levels in your scene. Go into the show->visualize->hdr view mode to see what the histogram looks like in your dark areas. If you cannot see any details in that view mode, and the histogram is basically empty, it means that the eye adaptation is not considering light levels that low, and you need to reduce the histogram low value in the post process volume until those details come back.