Excel overwriting changes (SharePoint Online/OneDrive) without prompting

Hi,

I have an usual one here and wondered if anyone had seen something similar.  For reasons I won't bore you with the "Autosave" feature in Excel is disabled (registry entry) and the files live in a SharePoint document library that is sync'd with OneDrive, these are Windows 10 PCs with the latest Office 365 clients.

User 1 opens a Excel workbook and starts work on cell row 1 and populates cells A,B,C, saves and exits the workbook.  User 1 then opens the same workbook and changes the value of B1 but at this point doesn't save the change.  User 2 opens the same workbook, sees the notification that User 1 is working on the same workbook.  User 2 makes a change call B1 and saves/exits, no error messages etc.  Then User 1 exit's and saves, again no warnings.  User 2 opens the workbook again and the edit they made has gone and they can see the edit that User 1 made. 

This doesn't seem correct, this could mean that one or the other use could potentially loose work with out evening knowing.  I was under the impression that if a file changed and changes clashed it would prompt the user.

Now this might be a consequence of the disabled auto-save but I need to go back to the user with a constructive argument on what is happening and how to stop it happening.

Thanks

Rob