The issue

If there is a metabox in the sidebar area, you can drag it into the larger area below the content area. However, once you have dragged metaboxes out of the sidebar, there is no way to drag any of them back into the sidebar unless you move them back from the Classic Editor interface. The same applies to moving all the metaboxes out of the area below the main content of the post.

Still an issue in Gutenberg 7.2.0.