Open files reverting to read only

Got a problem I am struggling to resolve.

When opening a file from our file server it opens correctly with read/write access. This is confirmed when checking in the "open files" mmc.

After leaving the file open for roughly half a day, it will complain that the file is open as read only when trying to save. The only way to save is to save it under a different name.

Checking under open files it confirms that the same user still has read/write access to the file. If I close the open file through the open files mmc it will then let the user save the file again.

File server is Windows 2012 R2. We are using DFS to access the shares. Only one DFS target is being used, not using DFSR for any of the shares.