This is because TABs do not change how Viewpoints work. They just offer a new way of arranging them in your application window.
A TAB is simply the given Window being pinned to the TAB bar instead of floating freely.
The Floor Plan Viewpoint and the 3D Viewpoint still have one Window each (all Stories in the same Window, both 3D Perspective and 3D Axonometry in the same Window). All other Viewpoint types, which are generated types, can have multiple Window instances (multiple Sections, Elevations, Details, Worksheets, 3D Documents, etc).
In summary, to be able to implement this the basic way of how ARCHICAD viewpoints and windows work would have to be changed.
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