Preview with code formatting only works once

I start a new post or comment in the forum and add some code to it using @-tags. I click on the preview button, the preview is rendered correctly (see Preview-1.png). I then edit the text further, hit preview again, then the code is just formatted with monospace font, but no syntax highlighting etc. is done (see Preview-2.jpg)

The same happens if I hit preview the first time without any code entered. If I add some code afterwards and hit preview the second time, no pretty formatting is done.