Apple has confirmed that the new 12.9-inch M1 iPad Pro (which is 0.5mm thicker than the previous-generation iPad Pro) doesn’t close all the way when using an older Magic Keyboard.
In a Magic Keyboard support document, Apple says that while the first-generation Magic Keyboard is “functionally compatible” with the fifth-generation iPad Pro, it’s “possible” the keyboard won’t precisely fit when closed.
The first generation of the Magic Keyboard (A1998) is functionally compatible with the new iPad Pro 12.9-inch (5th generation) with Liquid Retina XDR display. Due to the slightly thicker dimensions of this new iPad Pro, it’s possible that the Magic Keyboard may not precisely fit when closed, especially when screen protectors are applied.
Users of screen protectors will find that the closing issue is a bit worse.
Note that this issue only impacts the 12.9-inch model as the 11-inch iPad Pro is the same thickness as the previous generation and is fully compatible with the original Magic Keyboard.