Displaystyle="true" does not apply to all elements within an mrow

Issue Summary

When mrow is given the attribute displaystyle="true" it should apply to all elements within said mrow. This does not happen with MathJax.

Both of the summation expressions are nested inside an mrow element with the attribute displaystyle="true". Visually both of them should appear as uppercase symbols (as if they were block equations), where the numbers 5 and 6 should be under them, not as subscripts.

MathJax 4 output:

Firefox's native output:

OS: Windows 11
Browser: Mozilla Firefox 153.0

Steps to Reproduce:

Use the code to render:

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow displaystyle="true">
  <munder>
    <mo>∑</mo>
    <mn>5</mn>
  </munder>
  <munder>
    <mo>∑</mo>
    <mn>6</mn>
  </munder>
  </mrow>
</math>

Technical details:

• MathJax Version: 4
• OS: Windows 11
• Browser: Mozilla Firefox 153.0

I am using the default MathJax configuration and loading MathJax via

<script src="https://cdn.jsdelivr.net/npm/mathjax@4/tex-mml-chtml.js" defer></script>

Start with the MathJax 4 MathML rendering path for mrow, displaystyle, and munder, using the HTML reproduction in the issue with the default tex-mml-chtml.js configuration. Compare the rendered result with Firefox's native MathML output; done means both summation symbols render in display style with 5 and 6 underneath rather than as subscripts.