Why does Visio "Auto Size", when it expands the size of the foreground page downward, seem to shift background page objects upward?

Question

Steps to reproduce:

1. Create new Basic Diagram (US Units) document
2. Change Page-1 to Type "Background" (Right-click page tab > Page Setup...)
3. Add Basic Shape to Page-1 (anywhere; default size OK)
4. Add Page-2 (leave as default Type "Foreground")
5. Set Page-2 background to Page-1 (Right-click page tab > Page Setup...); you will see the background shape, from Page-1, appear on Page-2, as expected
6. Add first Basic Shape to Page-2 (anywhere; default size OK)
7. Add second Basic Share to Page-2, below the first, so Auto Size expands the size of the current page downward
8. Select Page-1

It seems Visio has now shifted the Page-1 background object off the background page, so it floats above and off the background page.

What's going on here...is this expected behavior? Or a bug?