The D100 most definitely CAN be set to a specific ISO equivalent and left there.
The D100 does have, as you have surmised, a custom function, that, when enabled, lets the camera change the ISO equivalent to get a proper exposure IF there is no adjustment left in the shutter speed or aperture, depending on the exposure mode. In the manual mode, the shutter speed and aperture stay as set, and the ISO equivalent will be changed to keep proper exposure.
When the custom function is disabled, ISO is fixed at whatever the user set it at. Nothing unusual at all.