TOPIC:

One possibility: Each view window has its own visibility settings, so its possible to switch of the visibility of the windows in the 3D view yet still see them in other views. Changing the view status in one view does not affect the other views.
To activate the Visibility state for a 2D or 3D view window, right click on the current active view window and from the context menu select Visibility. The visible categories are then displayed. In this example ensure that Extension elements - Window is enabled.

Also: If the object still do not show, you can delete the 3D view and create a new 3D view. (the 3D view is created from the 2D plan.

See also: Visibility

Why does my 2D view with sliding doors not show those doors on the 3D view (even though you can see them on the Section view)?

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