Windows 2012 server and permissions issues

Hi. I have issues with changing permissions using and iMac when a Microsoft 2013 Office Word file is saved to a share on a Windows 2012 Standard server.

· Word file can be opened, edited, saved and closed by Windows PC users with no problem.

· If the Word file is opened by an iMac or MacBook running Yosemite or Mavericks with Mac Office 11 and then edited and saved, the file permissions are altered so that Windows PC users can no longer open the Word file.

· The iMac or MacBook are connected to the same domain and are authenticated to the Windows 2012 Standard server. Protocols used are AFP and SMB.

Thanks in advance for any help.