JSFiddle - React, Tailwind, and code Playground

HTML

<div id="flex-container">
  <div id="flex-item-1" class="flex-item">
    flex item 1
    <div class="div-under-item-1">div under item 1</div>
  </div>

  <div id="flex-item-2" class="flex-item">
    flex item 2
  </div>
</div>
<input type="button" onclick="removeZIndex();" value="remove z-index from flex container">
<input type="button" onclick="addZIndex();" value="add z-index to flex item 1">

CSS

#flex-container {
  position: relative;
  z-index: 1;
  display: flex;
  justify-content: space-around;
  width: 600px;
  height: 250px;
  background-color: lightgray;
  margin-bottom: 40px;
}

.flex-item {
	width: 200px;
	height: 200px;
}

#flex-item-1 {
  position: relative;
  background-color: aqua;
}

#flex-item-2 {
	background-color: lightgreen;
}
  
.div-under-item-1  {
  position: absolute;
  z-index: -1;
  width: 200px;
  height: 150px;
  left: 150px;
  bottom: -70px;
  background-color: red;
  padding: 0 30%;
  box-sizing: border-box;
  text-align: center;
}

JavaScript

function addZIndex() {
  document.getElementById("flex-item-1").style.zIndex="1";
}

function removeZIndex() {
  document.getElementById("flex-container").style.zIndex="auto";
}