This might shed some light

If it doesn't pass the tests, it doesn't return the right answer. No way it can be different: that's what tests do: compare your output with the expected one and display the corresponding message if they don't match.

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The code you provide is incorrect, it throws a SyntaxError.

I strongly advice you to read carefully this: https://docs.codewars.com/training/troubleshooting/

You don't test the right input.

Remove those calls to your function at the end of your code, that's what's being detected as recursion, not set or repeat.

The index should be in the range [0..length-1]. If it is not, GetNth() should throw/raise an exception

Your code is filtering the mutated list instead of the original one, that's why you get the wrong order.

If the problem is indeed with the returned items' order, you're sorting them incorrectly. I assume your solution relies on Array.prototype.sort to be stable which isn't true here. Either way, apart from the issues reported above, everything works correctly. Not an issue.

Are you mutating the input? Probably yes, don't do it.

indexOf returns the first found index of the neccessary letter;
Since there are a lot of same letters in the long string tests, indexOf returns wrong coordinates of the neccessary letter