Door Tags of Linked revit model retrieve null value

Why do Door Tags of Linked Revit Model retrieve null value, while Room Tags still get accessible value when we use Select Model Elements?

If that cannot change because of the system, How can we access those Door tags?
(Door tags are in the main File, but their hosts (Doors) are in the Linked File)

@Jin ,

… a i see linked Elements

there you can use bimorphnodes



My door tags are in the main file, just my doors are in the linked file. But the door tag, when I select, show null value.
I does not mean the doors or other elements. I mean door tags only. Because other elements and other tags such as Room tags still show correct information (Room elements are also in linked model).