Links default to display:block;

You can add this to the container to solve:

.MyFlexBlockContainer a {
  display:contents;         /*Stop links addding default display:block property and killing flexblock layout*/
}

Or you can set it to the link instead:

  <a href="/some-page" style="display: contents" >
    <div class="MyFlexObject">

    </div>
  </a>