Ok, I improved the CSS to include mdb-select with md-outline.

Please check also my open issue on mdb-select for the related javascript.

This is the final version:

.md-form {

  .prefix {
    color: $input-label-color;
  }

  &.md-outline {

    .select-wrapper {

     + label {
        z-index: 0;

        &.active {
          color: map-get($data, skin-accent);
        }
      }
    }

    .caret {
      right: 0.5rem;
      color: $input-label-color;
    }

    input[type=text],
    input[type=password],
    input[type=email],
    input[type=url],
    input[type=time],
    input[type=date],
    input[type=datetime-local],
    input[type=tel],
    input[type=number],
    input[type=search-md],
    input[type=search],
    textarea {

      ~ label.active {
        color: map-get($data, skin-accent);
      }
    }

    input[type=text].form-control.valid + label:after,
    input[type=text].form-control:focus.valid + label:after,
    input[type=password].form-control.valid + label:after,
    input[type=password].form-control:focus.valid + label:after,
    input[type=email].form-control.valid + label:after,
    input[type=email].form-control:focus.valid + label:after,
    input[type=url].form-control.valid + label:after,
    input[type=url].form-control:focus.valid + label:after,
    input[type=time].form-control.valid + label:after,
    input[type=time].form-control:focus.valid + label:after,
    input[type=date].form-control.valid + label:after,
    input[type=date].form-control:focus.valid + label:after,
    input[type=datetime-local].form-control.valid + label:after,
    input[type=datetime-local].form-control:focus.valid + label:after,
    input[type=tel].form-control.valid + label:after,
    input[type=tel].form-control:focus.valid + label:after,
    input[type=number].form-control.valid + label:after,
    input[type=number].form-control:focus.valid + label:after,
    input[type=search-md].form-control.valid + label:after,
    input[type=search-md].form-control:focus.valid + label:after,
    input[type=search].form-control.valid + label:after,
    input[type=search].form-control:focus.valid + label:after,
    textarea.md-textarea.form-control.valid + label:after,
    textarea.md-textarea.form-control:focus.valid + label:after {
      top: 3.75rem;
    }

    input[type=text].form-control.invalid + label:after,
    input[type=text].form-control:focus.invalid + label:after,
    input[type=password].form-control.invalid + label:after,
    input[type=password].form-control:focus.invalid + label:after,
    input[type=email].form-control.invalid + label:after,
    input[type=email].form-control:focus.invalid + label:after,
    input[type=url].form-control.invalid + label:after,
    input[type=url].form-control:focus.invalid + label:after,
    input[type=time].form-control.invalid + label:after,
    input[type=time].form-control:focus.invalid + label:after,
    input[type=date].form-control.invalid + label:after,
    input[type=date].form-control:focus.invalid + label:after,
    input[type=datetime-local].form-control.invalid + label:after,
    input[type=datetime-local].form-control:focus.invalid + label:after,
    input[type=tel].form-control.invalid + label:after,
    input[type=tel].form-control:focus.invalid + label:after,
    input[type=number].form-control.invalid + label:after,
    input[type=number].form-control:focus.invalid + label:after,
    input[type=search-md].form-control.invalid + label:after,
    input[type=search-md].form-control:focus.invalid + label:after,
    input[type=search].form-control.invalid + label:after,
    input[type=search].form-control:focus.invalid + label:after,
    textarea.md-textarea.form-control.invalid + label:after,
    textarea.md-textarea.form-control:focus.invalid + label:after {
      top: 3.75rem;
    }
  }
}

Thanks @gianlucagiacometti for your suggest. We're going to include this idea soon.

Moreover, the value of "top" for all elements should be 3.75rem, and not 2.75 rem.

WHATEVERINPUT.form-control.valid + label:after,
WHATEVERINPUT.form-control:focus.valid + label:after {
    top: 3.75rem;
}

WHATEVERINPUT.form-control.invalid + label:after,
WHATEVERINPUT.form-control:focus.invalid + label:after {
    top: 3.75rem;
}