ESLint flat config(TS) - It scans unincluded files

I'm using ESLint with typescript and wrote the eslint.config.mjs as below.

import importPlugin from "eslint-plugin-import";
import js from "@eslint/js";
import ts from 'typescript-eslint';

export default ts.config({
    files: ["src/**/*.ts"],
    extends: [
        js.configs.recommended,
        ...ts.configs.recommended,
        importPlugin.flatConfigs.recommended,
        importPlugin.flatConfigs.typescript,
    ],

    languageOptions: {
        ecmaVersion: 5,
        sourceType: "script",

        parserOptions: {
            tsconfigRootDir: ".",
            project: ["./tsconfig.json"],
        },
    },

    rules: {/* omitted here because it's so long */}
});

Running eslint . with this configuration results in the following error:

<PROJECT-ROOT>/built/index.js
  <row>:<col>  error    Definition for rule '@typescript-eslint/no-unnecessary-condition' was not found           @typescript-eslint/no-unnecessary-condition
  <row>:<col>  warning  Unused eslint-disable directive (no problems were reported from 'no-constant-condition')

(and non-problematic warnings for src/**/*.ts)

The problem is it's linting ./built/index.js that's not included in files field in the configuration. It's value is ["src/**/*.ts"] and should be meaning ESLint should scan only .ts files in src folder.

Error message itself is quite understandable because ESLint is scanning .js file and @typescript-eslint/no-unnecessary-condition is not for this.

Additional info: this project has some .js or .mjs files outside ./src or ./built, and they do not seem to be scanned.

Additional info 2: adding ignores: ["built/"] did not work. "built/*", "built/**/*", "built/index.js" resulted in the same.

From https://eslint.org/docs/latest/use/configure/configuration-files#specifying-files-and-ignores:

You can use a combination of files and ignores to determine which files the configuration object should apply to and which not. By default, ESLint lints files that match the patterns **/*.js, **/*.cjs, and **/*.mjs. Those files are always matched unless you explicitly exclude them using global ignores.

export default ts.config(
  {
    ignores: ["**/*.js"]
  }
  {
    files: ["src/**/*.ts"],
    // ...
  }
);