[building-rollup] How to force the modern build to convert the optional chaining?

[abdonrd]

Using the @open-wc/building-rollup with legacyBuild: true…
The legacy build has the optional chaining converted by Babel.

How can I force the modern build to convert the optional chaining too?

I have tried to create a babel.config.json with:

{
  "plugins": ["@babel/plugin-proposal-optional-chaining"]
}

And also with this in the createSpaConfig:

babel: {
  plugins: ['@babel/plugin-proposal-optional-chaining'],
},

But neither option has worked for me.