Here's a slightly more minimal example: $cong— *italics* cong$: code
`$cong— *italics* $cong$.  The lack of a space between the MathJax and
em-dash is necessary ($cong$ — *italics* $cong$: code
`$cong$ — *italics* $cong$`), and there also must be MathJax on both
sides ($cong$— *italics*: code `$cong$— *italics*`).