This issue doesn't seem to have anything to do with tags, especially since it's replicable here on MSE where virtually no language hints have a tag with the same name.

Instead, like you mentioned in your edit, it seems like we've got two distinct issues here:

1. Highlighting in post previews is broken, possibly because it wasn't updated to use the new rules now applied to actual, rendered posts.

   E.g. using lang-nonexistent on a post still produces highlighting in preview:

   

   Despite this not doing anything in rendered posts:

   console.log('Hello highlighter!'); 

2. Many aliases are now completely broken, and do not produce any highlighting. E.g., for JavaScript there are 5 aliases:

   lang-javascript, lang-js, (lang-jsx, lang-mjs, lang-cjs)

   But only the lang-js appears to actually work when posted:

   lang-javascript

   console.log('Hello highlighter!'); 

   lang-js

   console.log('Hello highlighter!'); 

   lang-jsx

   console.log('Hello highlighter!'); 

   lang-mjs

   console.log('Hello highlighter!'); 

   lang-cjs

   console.log('Hello highlighter!'); 

   Despite all of the above working in preview: