MathML: fractions in square barckets not rendering in PDF

An equation that has a fraction inside of square brackets fails to render properly when I create a PDF.

Here is the MathML code:

<math>

   <mrow>

      <mo>[</mo>

      <mfrac>

         <mrow>

            <mn>1</mn>

         </mrow>

         <mrow>

            <mn>2</mn>

         </mrow>

      </mfrac>

      <mo>]</mo>

   </mrow>

</math>

It looks as it should in the MathFlow editor and Frame WYSIWYG editor, like this:

But when it is saved as a PDF it looks like this:

By the way, if I use the same code but use parentheses instead of square brackets, it looks fine. If I have other things in the brackets (not fractions) they also render fine in PDF.

I am using FrameMaker 2017 ver. 14.0.2.425 and MathFlow ver. 2.0c.

Thanks for your help.