If you want to use newlines in a MathJax expression, please use Shift+Enter instead of just Enter, as a normal newline will prevent MathJax from working correctly.

An alternative to waiting for the fix would be to use the MathJax shortcuts instead. If you press ctrl+m then e, you’ll open the MathJax editor and will get a live preview as you type, and shift+enter will work correctly in there.