Hi Doug,
Thanks for your detailed explanation and screenshots.
Seems Word co-authoring takes no effect causing this issue. You could follow the steps below to delete cache files to check if it helps:
1.To open the Office Upload Center, do one of the following, depending on your version of Windows:
- In Windows 10, click the Windows button, type Microsoft Upload Center in the Search box, and then double-click the Microsoft Upload Center app in the search results (it may say 2010 or 2013, depending on your Office version).
2.In the Upload Center, click Settings, and then click Delete Cached Files.
For more details, please refer to Delete your Office Document Cache.
In the meantime, please check if the document is checked outon one of the users' ends.
If the issue persists, I would like to confirm the following information to narrow down this issue:
1.Does the issue only occur in the specific document? How about other documents in the library?
2.Please share both users' Word version (Word >File >Account) and OneDrive version (Click OneDrive icon >Settings >About) with us to try reproducing this issue on our end.
Regards,
Eli