Overbrace does not appear in docx output

[jgm]

Overbrace does not appear in docx output

[jgm]

Let's try the simplest case.

pandoc -o my.docx
$$\overbrace{x+y}$$

Even here I don't see the overbrace.
texmath -t omml lets you see the omml produced by texmath:

<m:oMathPara>
  <m:oMathParaPr>
    <m:jc m:val="center" />
  </m:oMathParaPr>
  <m:oMath>
    <m:acc>
      <m:accPr>
        <m:chr m:val="︷" />
      </m:accPr>
      <m:e>
        <m:r>
          <m:rPr />
          <m:t>x</m:t>
        </m:r>
        <m:r>
          <m:rPr />
          <m:t>+</m:t>
        </m:r>
        <m:r>
          <m:rPr />
          <m:t>y</m:t>
        </m:r>
      </m:e>
    </m:acc>
  </m:oMath>
</m:oMathPara>

I created the equation in Word and it produced:

<m:oMathPara>
        <m:oMath>
          <m:groupChr>
            <m:groupChrPr>
              <m:chr m:val="⏞"/>
              <m:pos m:val="top"/>
              <m:vertJc m:val="bot"/>
              <m:ctrlPr>
                <w:rPr>
                  <w:rFonts w:ascii="Cambria Math" w:hAnsi="Cambria Math"/>
                  <w:i/>
                </w:rPr>
              </m:ctrlPr>
            </m:groupChrPr>
            <m:e>
              <m:r>
                <w:rPr>
                  <w:rFonts w:ascii="Cambria Math" w:hAnsi="Cambria Math"/>
                </w:rPr>
                <m:t>x+y
                </m:t>
              </m:r>
            </m:e>
          </m:groupChr>
        </m:oMath>
</m:oMathPara>

[jgm]

Okay, I've determined that the problem is the character we're using for the overbrace.
We're using ︷. Changing this to ⏞ fixes things.