bootstrap4.css
.align-self-stretch {
  -ms-flex-item-align: stretch !important;
  align-self: stretch !important;
}
■レイアウト:Flex d-flex:ユーティリティ:flexを交差軸上で個別に整列 align-self-stretch

flex文字01
flex文字02(align-self-stretch)
flex文字03
  <div class="d-flex" style="height: 100px">
    <div>flex文字01</div>
    <div class="align-self-stretch">flex文字02(align-self-stretch)</div>
    <div>flex文字03</div>
  </div>