Spacing issue involving single '=' in fractions in mrow

Created by: NSoiffer

Issue Summary

The spacing is different between these fractions is not symmetric (extra space before last fraction):

Note: this weird example came from looking at how some MathML spacing rules are being interpreted by different renderers. Since MathJax uses TeX rendering, I was quite surprised to see it doesn't seem to render this properly. Next I'll try turning on MathML spacing rules, where right and wrong are much more subjective ;-)

Steps to Reproduce:

With this header:

<script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
<script type="text/javascript" id="MathJax-script" async
  src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js">

this MathML produced the above input

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mn>1</mn>
  <mfrac>
	<mo lspace="0" rspace="0">=</mo>
	<mn>2</mn>
  </mfrac>
  <mn>1</mn>
  <mfrac>
	<mo>=</mo>
	<mn>2</mn>
  </mfrac>
  <mn>1</mn>
  <mfrac>
	<mn>2</mn>
	<mo>=</mo>
  </mfrac>
  <mn>1</mn>
</math>

The problem occurs in both Chrome and Firefox.

You can view this at https://codepen.io/nms/pen/MWwxEKj.