Comment 30 for bug 34813

also, I guess this could be related with:

from 'man smb.conf'
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 dos filetimes (S)
          Under DOS and Windows, if a user can write to a file they can change the timestamp on it. Under POSIX semantics, only the owner of the file or
          root may change the timestamp. By default, Samba runs with POSIX semantics and refuses to change the timestamp on a file if the user smbd is
          acting on behalf of is not the file owner. Setting this option to
           yes allows DOS semantics and smbd(8) will change the file timestamp as DOS requires. Due to changes in Microsoft Office 2000 and beyond, the
          default for this parameter has been changed from "no" to "yes" in Samba 3.0.14 and above. Microsoft Excel will display dialog box warnings
          about the file being changed by another user if this parameter is not set to "yes" and files are being shared between users.

          Default: dos filetimes = yes
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but changing dos filetimes option does not solve the problem