Here are 2 examples that demonstrate the problem very simply.
It’s the same very small and simple website, just using var in https://minify-var.netlify.app/, whereas it use const and let in https://minify-const-let.netlify.app/
Everything is explained in the website itself to see the original script and the one on CDN.
You will notice that var is minified whereas const & let are NOT.
Related post : [Support Guide] Does Netlify optimize every single asset my site uses? - #23 by hillary