By all accounts the German army was extremely professional

This is a myth. The German professional soldiers distanced themselves from taking responsibility for atrocities under their watch. Yet they let atrocities occur lest they lose their command. While the hardcore Nazi true-believers were buzz-fucking-saws through eastern Europe. The "professional" soldiers looked the other way or quietly participated. A few actually took action to oppose the Nazi state these were a tiny minority.

Much of US initial accounts of the war on the eastern front came from the testimony of these surviving German officers. Obviously they had a specific reason to paint themselves in a better light under allied captivity. There weren't anyone to interview from the Soviet Union as they were all dead. There wasn't Soviet documentation, photographs or film for the US to piece together the eastern front. Much of what the west has learned about the eastern front came from after the collapse of the Soviet Union where historians finally got to review Soviet documentation. All the "nice" Germans commanders went passively into a genocidal rampage all the same as the actively monstrous Nazi arch-fiends.