I guess technically, since for many years there was not an official treaty ending WW2 in respect to Germany, there was no internationally legal basis to recognize the Oder-Neisse border between Germany and Poland. It was a “provisional” border set at Potsdam, with an understanding that the official border would be ratified later. So, technically, the map was correct.
The Treaties of Paris in 1947 resolved territorial issues relating to all European Axis powers except Germany. Per Wikipedia, the final treaty was “Treaty on the Final Settlement with Respect to Germany.” What a great name for a peace treaty. It was signed in Moscow, USSR, on 12 September 1990, and it paved the way for German reunification on 3 October 1990. It also formally established the Oder-Niesse border between Poland and Germany.
That must have been the rationale since the Sov’s would not agree to a WWII peace treaty with Germany that did anything other than make permanent East Germany’s status. Good thing Obama wasn’t around in those days to surrender that to Russia.