Incorrect MCS index in airport command-line utility

In the past couple of months, the number of oddities affecting the Wi-Fi information provided by macOS and the CoreWLAN framework, especially in the latest MacBook Pro (MBP) 2018 is concerning, to say the least. We have read many stories about how Apple's hardware and software quality is declining. Whether that's true or not goes beyond the scope of this blog, nevertheless, the question remains: What's going on, Apple?