Firstly my fade in/out FX is not working. Second as my mouse scrolls on from one nav li to the next, the sub-menus shift slightly and change from a vertical orientation to a horizontal one.

For my first issue, I tried changing all display: none; code to opaque: 1 visibility: hidden.
For my second issue, I tried all combinations of transition delays and margin/padding/border adjustments. Nothing seemed to work.

.arizonia-regular {
font-family: "Arizonia", cursive;
font-weight: 400;
font-style: normal;      
}

html {
    scroll-behavior: smooth;
}

body {
    background-color: #170633;
    color: #cccccc;
    font-style: "roboto", sans-serif;
    margin: 75px;
}

.logo {
    position: relative;
    font-family: "Arizonia", cursive;
    font-size: 2.5rem;
    text-shadow: 1px 1px 2px black, 0 0 1em gray, 0 0 0.2em gray;
    color: white;
}

nav {
    border: 2px solid ;
    border-image: linear-gradient(to right, black, transparent);
    border-image-slice: 2;
}

nav ul {
    list-style: none;
    margin: 0;
    padding: 0;
    display: flex;
    justify-content: flex-start;
    font-family: "Arizonia", cursive;
    font-size: 1.5rem;
}
  
nav li {
    padding: 0 50px 0 5px;
        text-shadow: 1px 1px 2px black, 0 0 1em gray, 0 0 0.2em gray;
        color: white;
}
  
nav a {
    text-decoration: none;
    color: white;
}
  
nav a:hover {
    color: #666;
}
  
.sub-menu {
    position: absolute;
    top: 100%;
    left: 0;
    padding: 10px;
    border: 2px solid white;
    background-color: #170633;
    padding: 10px;
    list-style: none;
    margin: 0;
    transform: skewX(-20deg);
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.3s ease-in-out, visibility 0.3s ease-out;
    transition-delay: 0.3s;
}
  
.sub-menu li {
    position: relative;
    margin: 0;
    padding: 10px;
    transform: skewX(20deg);
}
  
.sub-menu li a {
    display: block;
    text-decoration: none;
    color: #ccc;
    transition: transform 0.3s ease-in-out;
}
  
.sub-menu li a:hover {
    color: #666;
}
  
nav li:hover .sub-menu {
    opacity: 1;
    visibility: visible;
    transition-delay: 0s;
}

.dropdown > li {
    position: relative;
}

.dropdown .sub-menu {
    opacity: 0;
    visibility: hidden;
    position: absolute;
    top: 100%;
    left: 0;
    transition: opacity 0.3s ease-in-out, visibility 0.3s ease-in-out;
}

.dropdown > li:hover .sub-menu {
    opacity: 1;
    visibility: visible;
}

.sub-menu li .sub-menu {
    top: 0;
    left: 100%;
}

.sub-menu li:hover .sub-menu {
    opacity: 1;
    visibility: visible;
}

.builds {
    text-decoration: underline;
    display: flex;
    justify-content: center;
    align-items: center;
}

.sub-menu:before {
    content: '';
    position: absolute;
    top: -10px;
    left: -10px;
    right: -10px;
    bottom: -10px;
    border: 2px solid #000;
    transform: rotate(45deg);
    z-index: -1;
}

.dropdown > li:hover .sub-menu,
.sub-menu li:hover .sub-menu {
    display: block;
}