Hi, thanks for sharing this. Are you referring to the Tags feature in OneNote or Outlook on your iMac?
If it’s OneNote, custom tags don’t sync between OneNote for Mac and OneNote Online. Microsoft only supports tag customization in the desktop app, and those tags stay local to that version. Make sure you’re signed in with the same Microsoft account, your notebook is stored in OneDrive or SharePoint, and the app is fully updated through Microsoft AutoUpdate.