UI: Navigation sections disappear after 5.3 upgrade

[elevatebart]

Describe the bug
Upgrading to 5.3 from 5.2.3 made the groupped components of the side menu disapear

To Reproduce
Open the repo
Run yarn storybook

Expected behavior
In a minor version we should not see a feature broken without backwards compatibility features

Code snippets

storiesOf("MyComp|02. General/MoreMenu", module)

should keep on yielding MyComp folder on the left

System:
Environment Info:

System:
OS: Windows 10 10.0.17763
CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
Binaries:
Node: 10.13.0 - C:\Program Files\nodejs\node.EXE
Yarn: 1.21.1 - ~\AppData\Roaming\npm\yarn.CMD
npm: 6.10.3 - C:\Program Files\nodejs\npm.CMD
Browsers:
Edge: 44.17763.831.0

[shilman]

Jiminy cricket!! I just released https://github.com/storybookjs/storybook/releases/tag/v5.3.3 containing PR #9425 that references this issue. Upgrade today to try it out!

Closing this issue. Please re-open if you think there's still more to do.