Should be fixed for good now!
Back-story: for technical reasons we are using two continuous integration services (CircleCI for PRs and Github Actions for pushes into master and hence for the .github.io website). The fix was applied for CircleCI but not for Github Actions.
Also: pydata-sphinx-theme released a 0.6.3 fixing the None bug you noticed so again thanks for making the scientific Python ecosystem better! Another side to the story, the person who did the release is Joris who appears in the MOOC teaser.