If you look at this question, you'll see that it's wrongly highlighted and the poster had to add
's in comments, so the whole code didn't show up as a big string.
The reason for this is that the code is highlighted using default highlighting, rather than using ML syntax highlighting as it should be (and would be on stackoverflow). Similarly lisp and clojure questions are also highlighted using the default highlighting rather than lisp highlighting. I'm sure there's more.
Since tag based syntax highlighting works correctly for other tags, I assume that while tag-based syntax highlighting is enabled, it doesn't use the same tag list as on SO. Maybe the list used here only contains the tags that already existed when the feature was first activated on this site?
So the list of tags used for syntax highlighting should be updated to contain the language tags that have been created since then. If this is technically possible, it should also contain tags for languages that are supported by prettify, but don't have a tag on this site yet. Ideally it would automatically use the same list as the one used on Stack Overflow.