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MathJax config: newlines in eqnarray contexts

In my answer to https://math.codidact.com/questions/278268 I have a couple of `eqnarray` contexts which are being rendered by MathJax but aren't being broken into lines as they should. This works fine in other sites with MathJax which I used to use, so I suspect that it's a problem with the configuration here. If I'm wrong, how should I alter my markup to make it work?

Tagged both bug and support because although I think it's the former it might turn out to be the latter.