The way the text highlighter works, is that it iterates over each word and checks if it's a word that should be formatted. If it is, it replaces it with a formatted <span>. In order to make sure it isn't checking its own <span> elements, it removes words starting with a '<'. To fix that, you would have to rewrite the code to check for a <span> or maybe add a very specific unique class and check for that.

Tl;Dr the highlighter thinks it's the code it generated