https://techcrunch.com/2018/07/19/apple-reportedly-confirms-keyboard-reliability-fix-in-internal-document/

An internal Apple document distributed to Apple Authorized Service Providers and obtained by MacGénération and MacRumors confirms that there’s a membrane under the keyboard to “prevent debris from entering the butterfly mechanism”. This is the first time Apple acknowledges that the third generation butterfly keyboard tries to fix unreliability issues.

“The keyboard has a membrane under the keycaps to prevent debris from entering the butterfly mechanism. The procedure for the space bar replacement has also changed from the previous model,” the internal document says.