I've added a publisher property to package.json to contain publisher metadata during the prepublish process. Npm appears to either be removing the property, or possibly using a cached version of the package data for publishing.
@KenanY pointed out that prepublish now runs after packing (tar'ing). I've also tried prepare and the same holds true for that as well.
npm info <module name>
npm install <module name>
npm config get registry
@shellscape "prepublish" runs after the package has been tarred. So you're adding that property too late.
@KenanY I'll append the OP. same thing happens with prepare