Well holy crap. In all my years of screwing up javascript, that's the first time I've heard of sort() being so unstable. Sure enough, it was an easy fix. Thanks a bunch!

It's a problem with your code, you're misusing sort. Use a better suited method for that. Sort is not stable in javascript.
Try your code in repl.it and you'll see it also fails there.

If you run the code that I've provided, you'll see that Kata is incorrectly showing something didn't pass, when it in fact DID pass. That's what I'm getting at there. Try my code in your browser console and it will pass 100% of the time. Try the exact same code in Kata, and it fails. This one isn't resolved.

Try:
isIsoGram("bncwxyfvesoriXljhmzputkgdaq");

Codewars returns: Expected: false, instead got: true
Console Returns : false

Not a kata issue:

JavaScript Completions 17225

I get 100% success when running tests, yet the submit always fails.

It doesn't rerun them when you submit.
Running sample tests are simply examples, to get the full test suite click 'attempt'.