JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js"></script>
$X_-$ (position ok)<br>
$X^-$ (position ok)<br><hr>

$X^a_-$ (subscript minus pulled upwards)<br>
$X^-_a$ (superscript minus pulled downwards)<br><hr>
    
${X}^a_-$ (position ok after curly braces added to "X")<br>
${X}^-_a$ (position ok after curly braces added to "X")<br>

JavaScript

MathJax.Hub.Config({
  jax: ["input/TeX", "output/HTML-CSS"],
  extensions: ["tex2jax.js"],
  tex2jax: {
    inlineMath: [['$','$'],['\\(','\\)']],
    displayMath: [['$$','$$'],['\\[','\\]']],
  },
});