There's a great video highlighting irregularities in AlphaStar MMR calculations and Blizzard matchmaking, notably, Alphastar, playing protoss, on average played with people 700 MMR below and victories for much weaker enemies gave unexpectedly huge boost to calculared MMR. When only matched with >6000MMR players, alphastar final had 6 win and 15 losses.
There is a problem with MMR calculation used for alphastar. More specific, there is matchmaking problem when alphastar did not get matched against equal-MMR and most of his protos games were against much lower-MMR players, skewing the data for MMR calculations.
The example given in video: you won 10 times against 5100, you lost against 7200. The calculated MMR would probably be around 6300. The problem is you wont be matched like this in real game. When matched mostly against the weaker enemies, MMR calculations have a lot of uncertainty.
The video, unfortunately in russian https://youtu.be/mpAUufSzaUo?t=1323