See end of this bug report for the true cause of the problem. It is the Editor that corrupts the initial post, but does not seem to affect subsequent edits. There are a few bug reports about the Editor and MathJax interacting incorrectly, this is a different one.
On the Operations Research site our MathJax FAQ renders newcommand defined \$\xrightarrow\$ (→) incorrectly compared to other sites with MathJax enabled.
Here is a screenshot from Operations Research:
Here is a screenshot from the Quantum Computing MathJax FAQ:
Notice "Example 1" does not render correctly but "Example 2" is correct.
This is what caused the MathJax to be displayed incorrectly, it's not a "MathJax error" but instead an "Editor error".
Copy pasting MathJax from another site and subsequently editing the first draft caused the editor to go inside the MathJax delimiters and alter numbers within the square brackets.
The editor should not go within \$$ or \$ delimiter pairs and alter numbers within square brackets when editing other sections of the post to add or remove links (which causes subsequent numbers within square brackets to be incremented, that must not occur within delimiters).