Children is undefined even though route matches

I'm experiencing an issue where I keep getting children = undefined in the parent's props when visiting a child route.

I'm using found with found-relay and I need to be able to visit routes matching /chat and /chat/12345. The following is part of my routing config:

...
{
  path: '/chat',
  Component: ChatView,
  query: graphql`...`,
  render: renderRoute,
  children: [
    { },
    {
      path: '/:threadId',
      Component: ThreadView,
      render: renderRoute
      query: graphql`...`,
    },
  ],
},
...

I am able to visit /chat just fine, and the page renders as expected. However, when I try to visit /chat/12345 I see the graphql request run, and the renderRoute function is called for the child route, however children is null for the parent ChatView component.

Now, if I modify this to use a named child as such:

...
{
  path: '/chat',
  Component: ChatView,
  query: graphql`...`,
  render: renderRoute,
  children: {
    thread: [
      {
        path: '/:threadId',
        Component: ThreadView,
        render: renderRoute
        query: graphql`...`,
      },
    },
  ],
},
...

Then I do see thread in the parent component's props, and child renders successfully.

What's going on here? Why doesn't the first example work?

Start with the route configuration for /chat and /chat/12345, then trace how renderRoute passes child routes into ChatView. Compare the unnamed child array with the named thread child and reproduce both cases. Done means explaining why the first configuration produces undefined or null children and documenting the supported configuration or behavior.