Bootstrap 3, Less: issue with changing placeholder color with & operator and multiple pseudo elements on the same line

Question

I'm trying to style the placeholder, and I noticed that placing multiple nested pseudo elements on the same line like this doesn't seem to work:

.form-group {
    .form-control{
        &::-webkit-input-placeholder, &:-moz-placeholder, &::-moz-placeholder, &:-ms-input-placeholder{
            color: @brand-primary;
        }
    }
}

Yet placing them separately like this does:

.form-group {
    .form-control{
        &::-webkit-input-placeholder{
            color: @brand-primary;
        }
        &::-moz-placeholder{
            color: @brand-primary;
        }
        &::-moz-placeholder{
            color: @brand-primary;
        }
        &:-ms-input-placeholder{
            color: @brand-primary;
        }
    }
}

I'm still learning Less, and I was under the impression that the first example was the right way to do things, so I was wondering what is going on here, and if I'm doing something wrong, can anyone point it out?