Users click command "Copy Link to this Page..." from Onenote Web App to create a shortcut link to a specific page in the notebook. When that link URL is used, it brings up the notebook but does not navigate to the specific page.
This affects both SharePoint online and on premise.
If the URL is greater than 255 characters this feature does not work, if less than 255 characters then all works.
Far from ideal, but if you shorten the name of the notebook section and page name then the URL will be shorter.
The product group has been made aware of this issue and it maybe resolved in a future version or patch.