The consensus appears to be that between 1 and 2 million ethnic Germans died when East Prussia and the "Recovered" Territories were ceded to the USSR and Poland after World War 2. If this is accurate, then this puts it in the same range of casualty and displacement as the other two mentioned above, which are the most oft-cited instances of chaos as a result of geopolitical division.
Because the Germans had just lost a a vicious war of conquest. No one was feeling sorry for them, and if they had made moves towards trying to recover those territorries, both the U.S. and the USSR would have stomped them again.