What you indicate may indeed be a bug, or (more likely) a display problem:
Your settings allow max real ISO of 100, since the original shutter was very slow to begin with custom autoiso would have wanted to increase ISO and get a faster shutter speed (closer to 1/500 if possible).
However, the ISO max you put in is LOWER than the camera suggested, and the real ISO was actually decreased to 100 (thus requiring a slower shutter. My guess is that the shutter was slowed (to around 1/5s) but due to rounding errors you did not see the effect on screen. Check the EXIF data to be sure.
The idea behind custom autoiso is to expand the range (say 75-400 market ISO) and provide more info (recommended shutter speed) to the camera to make intelligent decision by itself. Since you made the range smaller, the camera had little to do.
Also, try setting the base iso a little higher (mine is good at 5), anything lower is actually giving me worse results with slightly more noise.
Hope that helps,