Lazav and Nyxbloom Ancient bug

A player had Lazav, Dimir Mastermind on the battlefield, an opponents Nyxbloom Ancient was destroyed and Lazav copied the Nyxbloom. Then the Lazav player taps a basic land (generates 3 as expected), select undo, tap a different basic and it now produces 9 mana. This effect continues further where lands continue to tap for more and more.