There was an && instead of an ||, so it was checking if the setting to allow lactation to stop OR if they didn't have hyperlactation to allow them to stop lactating. Fixed in v0.9.4
One thing to note, though, the Hyperlactation will either increase the lactation to Hyperlactation if they are lactating already OR make them start lactating. So if it is applied to a non-lactating slave, you'll actually need to give her 2. The text for just lactation says something like "She's started lactating but it doesn't seem to be an abnormal amount"