Are you using Teams Education Edition?
When you create a staff team in Microsoft Teams, that team will come with its own OneNote Staff Notebook. A Staff Notebook is a digital notebook that helps facilitate educator collaboration at the school and district levels. Staff team creators own the notebook and have administrative permissions that allow them to add other members/co-owners and manage the notebook's settings.
The reason why your OneNote is forced to be renamed to Staff Notebook may be that the team owner has enabled Staff Notebook for your team.
The function of Staff Notebook is consistent with or even more complete than your original OneNote function. You can refer to this document to learn more.
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