在下面的代码块中， bootstrap 类d-none d-md-block正在取消d-flex.
<div class="col-md-6 col-sm-12 custom-right d-none d-md-block d-flex">
<h2 class="align-self-end">Title Here</h2>
</div>
在下面的代码块中， bootstrap 类d-none d-md-block正在取消d-flex.
<div class="col-md-6 col-sm-12 custom-right d-none d-md-block d-flex">
<h2 class="align-self-end">Title Here</h2>
</div>
如果我理解正确的话，你根本不应该用d-block.将其和d-flex替换为d-md-flex，以便仅将flex display应用于中等断点及以上.
见https://getbootstrap.com/docs/5.1/utilities/flex/#enable-flex-behaviors和https://getbootstrap.com/docs/5.1/utilities/display/#hiding-elements.