Be aware of the new NX1847 boundary selection menu!!! I just discovered that if you copy and paste a planer operation then delete the boundaries and select new, the Custom Boundary Data "DOES NOT" reset to default. Therefor, the values will continue on for every copy and paste planer milling operation until you discover the costly oversight on the floor. This did not happen in previous versions.
What you are discovering is not actually new to NX1847 but has been the behavior of the newer boundary dialogs for multiple releases now. I have been testing what you say in NX11 and have found the same behavior in operations where the dialog was given an upgrade. For example, if you created a Floor Wall or Cavity Mill operation in NX11 and created a trim boundary you will notice that the Custom Boundary Data such as your negative stock value will be retained when you copy/paste the operation and delete the previous boundary from the list.
It is true though that this is different behavior from the old boundary dialogs. When I select the Remove button in the Edit Boundary dialog from a copied Planar Mill operation is does indeed remove all previous boundary settings, including the Custom Boundary Data as you say.
In light of the fact that this behavior has been the standard way that the new boundary dialogs have worked since their initial release then this is not actually a PR but should be handled as an Enhancement Request (ER) if you want it to be changed.