Composition sidebar menu incorrect for title items with no grouping

[dexster]

Describe the bug
Composition menu items for referenced storybooks are displayed incorrectly when the title has no grouping ("/" separator). Instead of showing the menu item directly, the composition menu has an expansion icon and then shows 'Page'. In this image the composition menu is on the left and the referenced storybook menu is on the right.

To Reproduce
Create a composition storybook and have a ref to a storybook containing a story with a title with no "/"

export default {
  title: 'Getting Started System',
};

System
System:
OS: macOS 11.4
CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
Binaries:
Node: 14.15.5 - ~/.nvm/versions/node/v14.15.5/bin/node
Yarn: 1.22.10 - ~/.nvm/versions/node/v14.15.5/bin/yarn
npm: 6.14.11 - ~/.nvm/versions/node/v14.15.5/bin/npm
Browsers:
Chrome: 91.0.4472.114
Edge: 91.0.864.67
Firefox: 89.0.2
Safari: 14.1.1
npmPackages:
@storybook/addon-a11y: ^6.4.0-alpha.14 => 6.4.0-alpha.14
@storybook/addon-actions: ^6.4.0-alpha.14 => 6.4.0-alpha.14
@storybook/addon-essentials: ^6.4.0-alpha.14 => 6.4.0-alpha.14
@storybook/addon-links: ^6.4.0-alpha.14 => 6.4.0-alpha.14
@storybook/angular: ^6.4.0-alpha.14 => 6.4.0-alpha.14
@storybook/builder-webpack5: ~6.4.0-alpha.14 => 6.4.0-alpha.14
@storybook/manager-webpack5: ~6.4.0-alpha.14 => 6.4.0-alpha.14

[passbyval]

I'm seeing this issue as well on 6.5.0-alpha.47.

[passbyval]

Fixed by #17726.