Hello Paul Hulsbosch,
We're very sorry for the issue you are having. Class Notebook within Teams is integration features for Teams and OneNote, in my view we can use any one of them. If it doesn’t work as expected, this should be an issue. Since that sections can be opened directly through OneNote, then issue is more related with Teams app.
As I've tried but couldn’t reproduce this issue, may I confirm with you whether issue happens to all members in this Class team? And it occurs in both desktop as well as web app? Only specific sections can’t opened? If you’re using web app, try to use browser inPrivate mode, and also try to change a different browser to see if it can make any difference.
Also try to open OneNote in full mode and test it again:
Best Regards,
Anna