JSFiddle - React, Tailwind, and code Playground

HTML

<div id="flexWrapper">
    <ul id="flexContainer">
        <li class="flex-item">1</li>
        <li class="flex-item">2</li>
        <li class="flex-item">3</li>
        <li class="flex-item">4</li>
        <li class="flex-item">5</li>
        <li class="flex-item">6</li>
        <li class="flex-item">7</li>
        <li class="flex-item">8</li>
        <li class="flex-item">9</li>
        <li class="flex-item">10</li>
        <li class="flex-item">11</li>
        <li class="flex-item">12</li>
        <li class="flex-item">13</li>
        <li class="flex-item">14</li>
        <li class="flex-item">15</li>
        <li class="flex-item">16</li>
        <li class="flex-item">17</li>
        <li class="flex-item">18</li>
        <li class="flex-item">19</li>
        <li class="flex-item">20</li>
        <li class="flex-item">21</li>
        <li class="flex-item">22</li>
        <li class="flex-item">23</li>
        <li class="flex-item">24</li>
    </ul>
</div>

CSS

body, html {
    height:100%;
    margin: 0;
    padding:0;
}
#flexWrapper {
    display: flex;
    justify-content: center;
    background-color: aqua;
    height: 100%;
    align-items: center; /* This statement makes the problem */
    overflow: auto;
}
#flexContainer {
    width: 70%;
    list-style-type: none;
    padding: 0;
    margin: 0;
    display: flex;
    flex-direction: row;
    flex-wrap: wrap;
    justify-content: flex-start;
    align-items: center;
    align-content:flex-start;
}
li {
    background-color: tomato;
    border: 1px solid black;
    box-sizing: border-box;
    flex-basis:calc(100%/3);
    height:100px;
}