Samba force create mode does not seem to be enforced when running cp -a

Issue

I would like all clients to write files using 0777 permissions. To do this, I have edited my smb.conf with the following options:

[glustershare]
        ...
        vfs objects = glusterfs
        ...
        directory mode = 0777
        create mode = 0777
        force directory mode = 0777
        force create mode = 0777
        ...

When I copy a file (cp /file /share/file) to the (vfs-gluster) share, it has 0777 permissions as expected. However, when I copy a file with preserve permissions, (cp -a /file /share/file), the original mode is put on the file.