I can repro on Try. It looks like the autohighlight_all_code site setting isn’t being honored. The functionality appears to be enabled whether the setting is checked or not. The highlighted languages setting affect the language detection, but if everything is removed, it appears to default to csharp when it detects anything code-like.
The most recent core change that involved syntax highlighting appears to be @joffreyjaffeux’s refactor:
https://github.com/discourse/discourse/commit/f8062300da274cae559c17509e7c0d11458a065f