Gutenberg Block – Post Featured Image Filter Hook

From the documentation:

Prepend and append to the panel contents:

var el = wp.element.createElement;

function wrapPostFeaturedImage( OriginalComponent ) { 
  return function( props ) {
      return (
          el(
              wp.element.Fragment,
              {}, 
              'Prepend above',
              el(
                  OriginalComponent,
                  props
              ),
              'Append below'
          )
      );
  } 
} 

wp.hooks.addFilter( 
  'editor.PostFeaturedImage', 
  'my-plugin/wrap-post-featured-image', 
  wrapPostFeaturedImage
);

So did you intentionally not using wp.element.Fragment?

You may also want to check the example here:

• Gutenberg: Extending Featured Image Component