I have the 2020 MK and use it with my 2021 iPP 12.9". I had previously used the 2020 MK with a 2020 iPP.
As others report the differences in thickness isn't great. But, I found it discernible when I checked out the 2021 MK with my 2021 iPP. Others say using a screen protecter with the 2020 MK and the 2021 iPP is not a good idea, due to the tight fit.
One thing I did like was the 2021 MK's mounting magnets seemed stronger. Some have suggested my 2020 MK's magnets weaken over time. I don't know, but the 2021 MK really grabbed on to my 2021 MK.
In the end I did not keep the 2021 MK as I don't use the MK all of the time. So minor fit issues didn't really make the 2021 MK worth it for me as I already had the 2020 MK.
The other reason, not having to do with fit or function was the white 2021 MK (though it looks fabulous with my silver iPP) it seemed like it would be very, very hard to keep clean.
In fact I skinned my 2020 black MK, with a blue skin, due to cleanliness issues. Yes, I chickened out on the white color version. I only use the MK when I'm in a coffee shop, traveling, or craft beer pub ? So keeping the white MK clean seemed like it would be a constant hassle.
Finally, I thought if the 2022 iPP beckons me with better performance/looks/functionality, I doubt very much that the 2021 MK would work with a 2022 iPP. Note I pass my "used" Apple goodies down to family members/friends. Hey, if the 2022 iPP doesn't trip my trigger, then I bet I can get a 2021 MK for my 2021 iPP on even greater discount ?