How does the "position: sticky;" property work?

Check if an ancestor element has overflow set (e.g. overflow:hidden); try toggling it. You may have to inspect the DOM tree higher than you expect =).

This may affect your position:sticky on a descendant element.

Sticky positioning is a hybrid of relative and fixed positioning. The element is treated as relative positioned until it crosses a specified threshold, at which point it is treated as fixed positioned.
...
You must specify a threshold with at least one of top, right, bottom, or left for sticky positioning to behave as expected. Otherwise, it will be indistinguishable from relative positioning. [source: MDN]

So in your example, you have to define the position where it should stick in the end by using the top property.

html, body {
  height: 200%;
}

nav {
  position: sticky;
  position: -webkit-sticky;
  top: 0; /* required */
}

.nav-selections {
  text-transform: uppercase;
  letter-spacing: 5px;
  font: 18px "lato", sans-serif;
  display: inline-block;
  text-decoration: none;
  color: white;
  padding: 18px;
  float: right;
  margin-left: 50px;
  transition: 1.5s;
}

.nav-selections:hover {
  transition: 1.5s;
  color: black;
}

ul {
  background-color: #B79b58;
  overflow: auto;
}

li {
  list-style-type: none;
}

<nav>
  <ul align="left">
    <li><a href="#/contact" class="nav-selections" style="margin-right:35px;">Contact</a></li>
    <li><a href="#/about" class="nav-selections">About</a></li>
    <li><a href="#/products" class="nav-selections">Products</a></li>
    <li><a href="#" class="nav-selections">Home</a></li>
  </ul>
</nav>

I have same problem, and i found the answer here.

If your element isn't sticking as expected the first thing to check are the rules applied to the container.

Specifically, look for any overflow property set on any parents of the element. You can't use: overflow: hidden, overflow: scroll or overflow: auto on the parent of a position: sticky element.

Incase you came across this and your sticky is not working - try setting the parent to:

display: unset

Worked for me