Apple Authorized Service Providers have been distributed documents from Apple that confirm the presence of a membrane beneath the keys of the 2018 MacBook Pro models to prevent debris from entering the butterfly mechanism. While the membrane is for real, the reason for its existence is apparently not the same in all versions of the documentation.
https://www.notebookcheck.net/Apple-service-documents-confirm-presence-of-a-membrane-in-the-2018-MacBook-Pro-keyboards.317380.0.html