Weird .clone behaviour

class Mut {
    has @.a;
    method push($th) {
        push @.a, $th;
    }
}

class G {
    has Mut $.old;
    method dont_change_old {
        my $new = $.old.clone;
        $new.push(3);
        say "New: {$new.perl}";
        say "Old: {$.old.perl}";
    }
}

my $g = G.new(old => Mut.new);
$g.dont_change_old;
$g.dont_change_old;
$g.dont_change_old;

# OUTPUT:
# New: Mut.new(a => [3])
# Old: Mut.new(a => [])
# New: Mut.new(a => [3])
# Old: Mut.new(a => [3])
# New: Mut.new(a => [3, 3])
# Old: Mut.new(a => [3, 3])
#
# VERSION:
# This is Rakudo version 2017.01 built on MoarVM version 2017.01
# implementing Perl 6.c.