There could well be a bug here. If you are able, please could you perform this test:
- Go to the Edit quiz page for the problem quiz.
- Click one of the add/remove page-break icons.
- Press F5 to reload the page.
This will confirm if the edit really worked on the server.
If not, it would be an added bonus if you could
- Set Debugging to Developer level.
- And/or open the network tab in your web browsers developer tools.
- Then click the icon again.
Can you see the Ajax request in the browser developer tools, if so, does it look like it worked?
Are there any errors logged in the PHP error log on the server.
(Don't worry if you can't do the extra debugging, but it might reveal what is going on.)