It's an IXUS 145.
This is pretty odd. Shooting should not turn the override off, and does not on my cameras.
There is a shortcut to toggle MF, which I think will be UP in alt mode (while not in a menu etc) on your camera.
There is also a shortcut to disable overrides, which I think would be half press + down on your camera. This shouldn't change the SD override setting directly though.
If you have the CHDK alt help screen enabled (CHDK settings->menu settings->show <alt> help screen) it will show you what the various shortcuts are.
If the 'disable overrides on startup' option is set, the option will be turned off the next time you restart the camera.