Nesting slots is tricky, and the purpose of our Story objects is to illustrate how they work. In this case, the container object doesn't have a named slot, so providing a name confused the renderer and it didn't render the message as expected. This is now fixed.

web/src/elements/stories/LoadingOverlay.stories.ts

@@ -70,5 +70,5 @@ export const DefaultStory: StoryObj = {
 
 export const WithAMessage: StoryObj = {
     ...DefaultStory,
-    args: { ...DefaultStory.args, message: html`<p slot="body">Overlay with a message</p>` },
+    args: { ...DefaultStory.args, message: html`<p>Overlay with a message</p>` },
 };