in some szenarios we working with Word (just word not the other office Apps) in Co-Authoring we face a issue when open the Word document stored in Onedrive, Teams, SPO:
In English like: This document couln't not edited in Co-Authoring mode
Issue appears mostly if the person which opens the document first with the Word Client App and the secound User with the browser. Already known is the first User needs to activated Automatic Save. But if we test this with different users the behaviour is not the same for all of them.
Sometimes the secound user gets the message could not safe the file do you want to save it as a copy