Ok, I found what I was doing wrong: I did not wait for auto-focus and auto-exposure during the half-shutter press. Works fine. Thank you!
It's supposed to work in both cases, so this is probably a bug. There is a common bug like this, but the ixus115 code appears to already have the workaround
Can you check whether shutter overrides when you shoot without waiting for AF?
To test, half press to see what shutter speed the camera picks, use the CHDK "Enhanced Photo Operations" to override to something very different, then check the exposure of the resulting image.