I'm not sure about this, but I believe that Ixus 70 doesn't have a real iris diafragm but only the ND filter: without filter f/2.8; with filter (like it was) f/8
if camera shows f/8, it's going to use the ND filter. You can avoid this with CHDK and use a higher shutter speed to compensate.
When camera shows f/2.8, you can force it to use the ND. As you said, screen go darker and camera won't cahnge the shutter speed. Do it yourself with Tv override.
When I had some spare time, I'll test these things.