Babel not transpiling ES6 module in node_modules despite proper exclude

Question

In a react project I used react-boilerplate but had private modules to include in the frontend that needed transpiling. In order for babel to transpile those I set exclude to the following function in the webpack config that relates to babel:

rules: [
  {
    test: /\.js$/, // Transform all .js files required somewhere with Babel
    // eslint-disable-next-line object-shorthand, func-names
    exclude: function (modulePath) {
      return /node_modules/.test(modulePath) &&
          !/node_modules\/@trade-quorum\/tq-helpers/.test(modulePath);
    },
    use: {
      loader: 'babel-loader',
      options: options.babelQuery,
    },
  },

And it worked perfectly.

Now I used that same trick in a different project but this time, in the resulting bundle tq-helpers is included but not transpiled into ES5 - the ES6 code is directly in the bundle and the build raises an error (more specifically UglifyJS).

There must be a reason around the dependencies to this package that are not the same in the two projects but hard to find. I was wondering whether there is a way to debug in details what babel does for a specific package in order to find the reason.

Thanks you for your help,

Best,

Didier