Describe the bug
The blocks contained inside a Reusable Block aren't shown in the Block Navigation in a sub-tree similar to how other parent blocks such as Columns show the blocks they contain.

To reproduce
Steps to reproduce the behavior:

1. Insert multiple blocks in the Block Editor.
2. Highlight multiple blocks and select Convert to Reusable Block.
3. Name and Save the Reusable Block.
4. Open the Block Navigation from the Editor Toolbar and only Reusable Block is shown, not the contained blocks as a sub-tree.

Expected behavior
The Block Navigation should ideally reveal the contents of a Reusable Block in a sub-tree similar to how Columns, Groups, Media & Text and other blocks that contain children display a sub-tree when highlighted.

Additional context

• WordPress: Version 5.3.3-alpha-46995