How to make flex-basis work in IE11

My website is completely broken in IE11. The problem comes from flex: 1 1 0%;, which I use everywhere thanks to autoprefixer and postcss-flexbugs-fixes.

The site does work on IE when I change it to flex: 1 1 auto;, but then some behaviors change (e.g. one flexbox with two flex: 1 1 auto; children which do not take exactly the same space). Therefore this solution breaks my designs on other browsers (while making it a lot nicer - not broken - on IE11).

How do people manage to make their sites built with Flexbox work on IE11?

Edit: here is a pen which highlights the problem I am facing: https://codepen.io/Zephir77167/pen/GMjBrd (try it on Chrome and IE11).

Edit2: here is the code:

HTML:

<div id="a" class="flex">
  <div id="b" class="item flex-1">
    Hey
  </div>
  <div id="c" class="item flex-0">
    Ho
  </div>
  <div id="d" class="item flex-1">
    Heyheyheyheyho
  </div>
</div>
<br />
<div id="a" class="flex">
  <div id="b" class="item flex-1-variation">
    Hey
  </div>
  <div id="c" class="item flex-0">
    Ho
  </div>
  <div id="d" class="item flex-1-variation">
    Heyheyheyheyho
  </div>
</div>

CSS:

* {
  box-sizing: border-box;
}

#a {
  background-color: pink;
  height: 300px;
  width: 100px;
}

#b {
  background-color: green;
  height: 50px;
}

#c {
  background-color: blue;
  height: 100px;
}

#d {
  background-color: yellow;
  height: 150px;
}

.flex {
  display: flex;
  flex-direction: row;
  align-items: center;
  flex-wrap: wrap;
}

.item.flex-0 {
  flex: none;
}

.item.flex-1 {
  flex: 1 1 0%;
}

.item.flex-1-variation {
  flex: 1 1 auto;
}

When it comes to IE11, you could target it explicit using this CSS rule:

_:-ms-fullscreen, :root .IE11-only-class { 

  /* IE11 specific properties */

}

Stack snippet

* {
  box-sizing: border-box;
}

#a {
  background-color: pink;
  height: 300px;
  width: 100px;
}

#b {
  background-color: green;
  height: 50px;
}

#c {
  background-color: blue;
  height: 100px;
}

#d {
  background-color: yellow;
  height: 150px;
}

.flex {
  display: flex;
  flex-direction: row;
  align-items: center;
  flex-wrap: wrap;
}

.item.flex-0 {
  flex: none;
}

.item.flex-1 {
  flex: 1 1 0%;
}

_:-ms-fullscreen, :root .IE-FlexAuto {
  flex-basis: auto;
}

<div id="a" class="flex">
  <div id="b" class="item flex-1 IE-FlexAuto">
    Hey
  </div>
  <div id="c" class="item flex-0">
    Ho
  </div>
  <div id="d" class="item flex-1 IE-FlexAuto">
    Heyheyheyheyho
  </div>
</div>

Here is a post with an answer of mine, which talks some more about this, and also provide some script solutions which might be helpful