Currently, through a combination of 'Undo' actions, its possible to create a situation in which objects (in this case, Paths) appear on-Stage, but are absent from the Navigator.

Video attached.

Repro Steps:

1) Create a Project with a MainPage.xaml, and introduce a 256x256 Button to the Page.
2) Right-Click on the Button, and create a Template of it.
This will open the Template in the Template Dummy Editor.

3) Delete the default ContentPresenter from the Template.

4) Copy-Paste, or Import any SVG into the Root Border.
This will place the Path(s) within a Canvas.

5) Right-Click on the automatically-imported Canvas generated as part of the import, and from "Layout", select "Replace With-->Viewbox".
6) Select the newly-created Viewbox, and clear its Margins.

7) Press Undo once.
This will undo the Margin adjustment.

8) Press Undo again.
This, should in theory, undo the conversion of the Canvas containing Paths into a Viewbox containing paths.

Observe how instead, the Paths are deleted from the Scene, leaving a blank Canvas Element behind.

Expected Result: Undoing an Element Conversion from one Layout Element to another doesn't delete its contents.

9) Immediately import/paste the SVG asset again into the blank Canvas.

Observe how the SVG artwork appears on-Stage, but the Navigator is empty.
Observe how the SVG itself is not selectable on the Stage with either the Selection (V) or Direct Selection (A) tools

Expected Results: Any item present on-Stage should be present and visible in the Navigator.

10) Exit the Template through the 'exit scope' button in the lower left of the application window, then re-enter it again.

Observe how the Paths are now present in the Navigator as expected.

Note: This behaviour only occurs within Template, and does not seem to take place on for example, a regular Page.