tsconfig.json with allowJs: true does not downlevel JS

[please-rewrite]

I have the following tsconfig.json in a mixed project (js/ts):

{
  "compilerOptions": {
    "target": "es2019",
    "module": "commonjs",
    "moduleResolution": "node",
    "allowJs": true,
    "esModuleInterop": true,
    "strict": true,
    "outDir": "dist",
    "skipLibCheck": true,
    "sourceMap": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules"]
}

It seems like the ts transpiler is only being applied to the typescript files and the js files are taking a different path.

I only assume this because I have optional chaining in both the js and ts files and only the ts files get the proper conversion from ?. operation to chained null checks.