If your 3D is a saved View, that View also contains information about what elements to display in 3D.
Maybe it does not include the Mesh you created in the Floor Plan Viewpoint.
Activate the saved 3D View, then right-click and select the "Show All in 3D" command. If this is the cause, it should display that Mesh you created.
You would then need to resave the View to include those elements as well.
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