This issue comes from the the accessibility test performed by the Accessibility team (#20369). One of the issues raised was that navigating with a keyboard between parent and child blocks was difficult and unpredictable.

Moving between the main block and nested blocks was very difficult and unpredictable. There was no visible focus that gave a clear sense of place while exploring the blocks. It was very difficult to understand what was going - what mode am I in? Am I in the navigation block, or in a menu item?

As seen in the example above, while using the up/down arrows, the UI moves between edit and browse mode without this change being invoked by the user.