Steps to reproduce:

1. In the Site Editor, go to the Navigation page
2. Verify there is a a navigation item there, and its content is a Page List block, showing the list site's pages.
3. In the navigation's context menu, select Rename
4. Rename the navigation

Result: the navigation is not only renamed, but its content is also reset to empty string ('').

Here is a video:

Note that the "Sample Page" item (it's part of the Page List block) has disappeared after the rename.

The REST request to rename looks like this:

and this is the response:

Note that not only the title field has changed, but also the content has been reset.

@tellthemachines Maybe there is something wrong with the WP_Navigation_Fallback class in Core? If I understand it correctly, wp_navigation is just another custom post type, and its REST requests are handled by the vanilla posts controller. Editing individual fields should work without resetting the others. It's only WP_Navigation_Fallback that patches the schema somehow, including the content.* fields.