When you change the code snippet lang from original to something else it breaks

The language, css c plaintext, is not a valid language. Therefore, no Highlight.js language library exists that matches that language. You need to edit the source and remove the incorrect languages, leaving only the one language you want to be styled.

Incorrect languages that remain when switching the language is an issue with CKEditor 5. Not this module. This module only detects the languages, and applies the stylesheet from Highlight.js.

This issue is related to the Drupal core issue 🐛 [upstream] CKEditor 5 accumulates class properties when switch code language in Code Block Active and also as a CkEditor 5 issue https://github.com/ckeditor/ckeditor5/issues/15453.