Why Does Clip-Path (And Other Properties) Affect the Stacking Order (Z-Index) of Elements Later in Dom

Why does clip-path (and other properties) affect the stacking order (z-index) of elements later in DOM?

From the specifcation:

A computed value of other than none results in the creation of a stacking context the same way that CSS opacity does for values other than 1.

Then considering the painting order:

All positioned, opacity or transform descendants, in tree order that fall into the following categories:
All positioned descendants with 'z-index: auto' or 'z-index: 0', in tree order.
For those with 'z-index: auto', treat the element as if it created a new stacking context, but any positioned descendants and descendants which actually create a new stacking context should be considered part of the parent stacking context, not this new one.
For those with 'z-index: 0' treat the stacking context generated atomically.
All opacity descendants with opacity less than 1, in tree order, create a stacking context generated atomically.
All transform descendants with transform other than none, in tree order, create a stacking context generated atomically.

The element with clip-path is painted at the step (8) and the image will be painted before if has no position set

For all its in-flow, non-positioned, block-level descendants in tree order: If the element is a block, list-item, or other block equivalent ...

If you add position:relative to image it will be postioned and will fall under the step (8) and the tree order will decide making it above the clip-path element

Here is the same code with opacity where you will have the same painting order:

body {
  padding: 1em;
}

header {
  background: #a00;
  opacity:0.8;
}

h1 {
  margin: 0;
  padding: 2em;
  font: 300%;
  color: white;
  text-align: center;
}

section {
  background: #ccc;
  padding-top:5em;
  margin-top:-5em;
}

img {
  margin-top: -10em;
  
}

<header>
  <h1>Header Content</h1>
</header>

<section>
  <img src="https://via.placeholder.com/330/0000FF/808080"/>
</section>

CSS: backdrop-filter messing up stacking order

From the docs:

The backdrop-filter CSS property lets you apply graphical effects such
as blurring or color shifting to the area behind an element.

This means that the filter will not apply to the areas in front of the element. So for that just have a z-index property on your image with a non-static position

img {
  position: relative;
  z-index: 2; /* arbitrary value - the higher this is, the more the element will be on top of other elements */
}

CSS filter property disabling fixed position on navigation bar

Just add z-index to your nav element as follow

nav{
  position: fixed;
  background-color: #fff;
  z-index:999;
}
nav ul{
  margin: 0;
  padding: 0;
  }
.navbar-brand{
  padding-right: 20px;
}

nav li{
  display: inline-block;
  padding: 10px;
}
nav a{
  color: #000;
  text-decoration: none;
}
nav a:hover{
  color: #ff6600;
  text-decoration: none;
}

.title-image img{
  width: 100%;
  height: 300px;
  filter: brightness(60%);
}

<nav>
  <ul>
     <li>
       <a class="navbar-brand" href="#">Navbar Brand</a>
     </li>
     <li>
       <a href="#">Home</a>
     </li>   
     <li>
       <a href="#">About</a>
     </li>
     <li>
       <a href="#">services</a>
     </li>
  </ul>
</nav>


<div class="title-image">
  <img src="https://images.unsplash.com/photo-1599546824091-f49550ce8cbc?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=500&q=60">
</div>

Tooltip invisible after using backdrop-filter

This answer may help.

Why does clip-path (and other properties) affect the stacking order (z-index) of elements later in DOM?

I'd suggest removing the backdrop-filter from div.title, and add it to a pseudo-element instead.

div.tile {
  width: 400px;
  height: auto;
  display: inline-block;
  background: RGBA(24, 24, 19, 0.8);
  padding-bottom: 30px;
  /* REMOVE backdrop-filter: blur(4px); */
  position: relative; /* <-- ADD */
}

div.tile:before {
  content: '';
  backdrop-filter: blur(4px);
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  z-index: -1;
}

Why Does Clip-Path (And Other Properties) Affect the Stacking Order (Z-Index) of Elements Later in Dom