Co-Authoring: Paragraph not being locked with Word (Office 365 subscription) and SharePoint 2013

Hi, 

I have a customer that is using SharePoint 2013 with the latest version of Office 365 Office application client applications.

The customer just migrated from Office 2013 to an Office 365 subscription (not using SharePoint Online yet and there are no plans yet for a migration from SharePoint 2013 to SharePoint Online).

Previously, with Office 2013 and SharePoint 2013, when two or three users were working on the same document, local Word would automatically block other authors from making changes in a section/paragraph when one person was editing there. That no longer happens. As a result, every time a user hits save, they get a popup on the top saying there are conflicts. They then have to resolve conflicting edits.  I can see the other user listed as editing the document, but SharePoint (Office) is no longer locking a paragraph that is being edited.

We have made some tests using Word for Office 365 and SharePoint Online and paragraphs are blocked as expected. The problem is due to a conflict between the Office 365 version of Office the customer is now using and SharePoint 2013.

We also tested in the customer's SharePoint 2013 environment to co-author documents using Office 2013 version and it worked well, the paragraph was blocked.

Relevant version information:

SharePoint 2013: 15.0.4953.1000 (August 2017 CU)
Office (Office 365 subscription, co-authoring not blocking paragraphs): 16.0.10730.20102 64 bits
Office 2013 (co-authoring blocking paragraphs as expected): 15.0.5059.1000 64 bits

I appreciate help since the customer is evaluating moving files away from SharePoint to a file server.

Thanks,
Miguel