How can I stop the Breadcrumb control from setting SelectedItem to null when unloading?

Background:

We have an application that uses the Breadcrumb control. However, instead of displaying a file system hierarchy, we use it to navigate a visual hierarchy. In this case, 2D rectangles that can contain other rectangles.

Each rectangle has an ID. Thus, when a user selects an ID within the Breadcrumb control, our UI 'zooms in' on the ID's rectangle, so only that rectangle's nested child nodes are seen.

Problem:

When a user navigates away from our custom control, the breadcrumb control sets its SelectedItem property to null. Because we two-way bind to the SelectedItem property, to control the zooming capability, this has the unfortunate and unexpected effect of setting a ViewModel property to null.

Questions:

1. Is setting SelectedItem to null an intentional behavior, or a bug?
2. How can we stop that from happening?

Thanks,

-Craig