1 - Open a User form.
2 - Create a new reference field to the Incident table.
3 - Add the fields incident.category and incident.subcategory to the User form. Notice the dependency does not work.
This behaviour is as per system design. As a workaround, make the dot-walked fields read-only (i.e. through a UI Policy) on the referencing child form, and only update them on the parent record by clicking on the reference indicator.