Mac Spell checker is breaking version 16.9 of OneNote, please release a new version
We have fixed this issue in build 16.10. We are sorry for the inconvenience it has caused you. To get the update, go to the Mac App Store or if you installed it as part of Office, use these directions, Check for updates in OneNote for Mac. Let us know if you are still having issues.
Eric Schvimmer commented
This is a bug seemingly introduced with your last update. Please fix ASAP!
please this a big issue for many users