z-index not working on pseudo-element - css

Question

I have encountered a weird issue, while trying to implement a custom background on website I'm working on.
I've written a proof of concept code piece on codepen and it works perfectly there, but when I try to implement it on website it does not.
body {
background: black;
}
.background-test {
background: white;
width: 20%;
margin: 2% 50%;
height: 250px;
padding: 1%;
position: relative;
}
.background-test:before {
width: 100%;
height: 100%;
content: "";
background: red;
position: absolute;
z-index: -1;
bottom: -3%;
left: -2%;
-webkit-clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
}
<div>
<div class="background-test">Lorem Ipsum</div>
<div></div>
</div>
https://codepen.io/Hassansky/pen/eEaQxG/
You can see that the :after pseudo element is positioned correctly - in theory. When I try to implement this into the website I'm working on, it just doesn't show. Chrome Dev tools shows it's there, just not displaying it. It does show up when I disable the z-index on dev tools, but then it stacks on top of the parent, as it should.
I tried to look for stacking issues, but I'm at wits end and cannot find any reasonable explanation for this.
This is a Wordpress website loaded with theme, but this should not present an issue with z-index stacking, especially when I cannot find any rule regarding z-indexes there.
Page url: http://polkrys.pl/cukiernia/podklady-cukiernicze-okragle-biale/
I've marked which elements should have pseudo-elements on them.
I'm adding SASS code that relates to elements in question:
.polkrys_shadow {
position: relative;
&:before {
width: 100%;
height: 100%;
content: "";
background: red;
position: absolute;
z-index: -1;
bottom: -3%;
left: -2%;
-webkit-clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
}
The margins and paddings are defined within wordpress visual composer. I suggest inspecting mentioned elements with Dev Tools - you'll see it should work - but it doesn't.

Answer 1

The parent element of the :pseudo-element requires a z-index declared as well in order for the z-index of the :pseudo-element to function as intended. But doing so will stack the :pseudo-element over the parent element, concealing the background and nested content.
To rectify this, nested content should have a higher z-index declared. And an additional :pseudo-element (:after) should be declared to overlay the initial :pseudo-element (:before) with the background fill applied (e.g: "white"), to conceal the initial pseudo-element only allowing the overflow to reveal.
.logistic-bg .polkrys_shadow:after { /* Additional pseudo-element added */
content: "";
background: white;
position: absolute;
left: 0;
right: 0;
top: 0;
bottom: 0;
}
.logistic-bg .polkrys_shadow { /* Adjustments made on parent element */
z-index: 9;
}
.logistic-bg div:first-child { /* Adjustments made on nested element */
position: relative; /* required for z-index to apply */
z-index: 99;
}

Answer 2

I found this solution:
el {
transform-style: preserve-3d;
}
el:before {
transform: translateZ(-1px);
}

Answer 3

<div>
<div class="background-test">Lorem Ipsum <div style="z-index:100;color:blue;background-color:yellow;">vgghuu</div>
</div></div>
body {
background: black;
}
.background-test {
background: white;
width: 20%;
margin: 2% 50%;
height: 250px;
padding: 1%;
position: relative;
z-index: 1;
}
.background-test:before {
width: 100%;
height: 100%;
content: "";
background: red;
position: absolute;
z-index: -1;
bottom: -3%;
left: -2%;
-webkit-clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
}