We determine department using the users SharePoint account properties. I can access this with PnP JS, so I know it can be accessed by the JavaScript but just haven't found a way to filter the person lookup using this property.
Currently, the only option to filter the Person or Group field is to define a SharePoint group in the column settings. Thus, you can select only users from a specific group.
We can offer paid support for adding an ability to filter Person or Group field by account properties. If you are interested, email us at [email protected] for more information.