Bad Canonicalization for bad input

[NSoiffer]

This disaster of real world input:

<math xmlns="http://www.w3.org/1998/Math/MathML">
    <mi>S</mi><mi>I</mi><msup><mi>N</mi><mrow data-mjx-texclass="ORD"><mo>−</mo><mn>1</mn></mrow></msup>
    <mo>,</mo><mtext> </mtext>
    <mi>C</mi><mi>O</mi><msup><mi>S</mi><mrow data-mjx-texclass="ORD"><mo>−</mo><mn>1</mn></mrow></msup>
    <mtext>,</mtext>
</math>

ends up producing bad canonical MathML:

   <math>
    <mrow data-changed='added'>
      <msup>
        <mi data-maybe-chemistry='1' mathvariant='normal' data-split='true'>SIN</mi>
        <mrow data-mjx-texclass='ORD'>
          <mo>-</mo>
          <mn>1</mn>
        </mrow>
      </msup>
      <msup data-previous-space-width='0.7'>
        <mo>,</mo>
        <mi>COS</mi>
        <mrow data-mjx-texclass='ORD'>
          <mo>-</mo>
          <mn>1</mn>
        </mrow>
      </msup>
      <mo>,</mo>
    </mrow>
   </math>

Notice that the second <msup> has three arguments.