You need to add align-items:flex-start; to .square-link class and it will work as your expectation

I’m trying to have a drop-down in my menu, but because I’m using display: flex whenever I hover it also expands all boxes without drop downs, what’s the easiest way to fix this?

Also, is it possible to make it so that the drop-down boxes line up with the main header itself? Code pen below:

CSS:

.square-link
{
    z-index: 80;
    text-align: center;
    display: flex;
    flex-wrap: wrap;
    position: relative;
}

.square-link a
{
    border: 2px solid #373737;
    border-radius: 5px 5px 5px 5px !important;
    color: #373737;
    background-color: transparent;
    font-size: calc(7px + (26 - 18) * ((100vw - 300px) / (1600 - 300)));
    text-align: center;
    padding: 6px;
    padding-right: 2vw;
    padding-left: 2vw;
    margin-right: 1vw;
    margin-bottom: 3%;
}

.square-link a:hover, .square-link a.active
{
    background-color: #373737;
    color: #ffffff;
    transition: 0.2s
}

.dropbtn {
    border: 2px solid #373737;
    border-radius: 5px 5px 5px 5px !important;
    color: #373737;
    background-color: transparent;
    font-size: calc(7px + (26 - 18) * ((100vw - 300px) / (1600 - 300)));
    text-align: center;
    padding: 6px;
    padding-right: 2vw;
    padding-left: 2vw;
    margin-right: 1vw;
    margin-bottom: 3%;
  cursor: pointer;
  overflow: hidden;
  transition: 0.2s;
}

.dropdown {
  position: relative;
  display: inline-block;
  z-index: 75 !important;
}

.dropdown-content {
  display: none;
  position: relative;
  background-color: #f9f9f9;
  min-width: 160px;
  z-index: 100 !important;
}

.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
  z-index: 75 !important;
  border: none;
}

.dropdown-content a:hover {background-color: #f1f1f1}

.dropdown:hover .dropdown-content {
  display: block;
}

.dropdown:hover .dropbtn {
  color: white;
  background-color: #373737;
  transition: 0.2s;
}

.caret-down
{
    color: #373737;
}

HTML:

<link rel="stylesheet" href="//libapps-custom.library.curtin.edu.au/css/fontawesome.min.css?ver=5.8.11">
<div class="contentbox">
    <div class="square-link">
        <a href="">Header One</a>
<div class="dropdown">
  <button class="dropbtn">Header Two<i class="fa fa-caret-down"></i></button>
  <div class="dropdown-content">
  <a href="">1</a>
  <a href="">2</a>
    <a href="">3</a>
  <a href="">4</a>
  <a href="">5</a>
  </div>
</div>
 <a href="">Header Three</a>
<a href="">Header Four</a>
<a href="">Header Five</a>
    </div>
</div>

https://codepen.io/adms2000/pen/bGVJjNa