Steps to reproduce:

- Enable client-side MathJax in https://de.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering
- Enable "Vorschau sofort anzeigen" in https://de.wikipedia.org/wiki/Spezial:Einstellungen#mw-prefsection-editing
- Go to a non-existing article, https://de.wikipedia.org/w/index.php?title=grlglkff&action=edit&redlink=1
- Enter a formula in <math> tags, e.g. <math>x=2</math>.
- Click "Preview". (Notice that the formula is rendered properly.)
- Click "Preview" once again. (Notice that the formula is displayed as "$ x=2 $".)