Eu descobri o problema que tive para especificar o gid e o uid. Teria economizado tanto tempo se eu apenas lesse a página de manual com mais cuidado ...
uid=arg sets the uid that will own all files or directories on the
mounted filesystem when the server does not provide ownership information. It may be specified as either a username or a numeric uid. When not specified, the default is uid 0. The mount.cifs helper must be at version 1.10 or higher to support specifying the uid in non-numeric form. See the section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for more information.
gid=arg sets the gid that will own all files or directories on the
mounted filesystem when the server does not provide ownership information. It may be specified as either a groupname or a numeric gid. When not specified, the default is gid 0. The mount.cifs helper must be at version 1.10 or higher to support specifying the gid in non-numeric form. See the section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS below for more information.