There was a lot of disputes between Polan and Germany, Lithuania, and a few other countries after 1918-1919 period. There were disputes with Germany into the thirties. Some land was divided between Germany and Polan. Some land Germany did not relinguish until long after WW1. There were disagreements with Prussia and Polan.
Russia and Polan had several disputes about lands on their border. So the land you are talking about could have been under German control for a long time after WW1.
Go to the internet and type in Map of Polan Polan 1918-1935. Look for Wipkidia. I might have it spelled wrong. There are a lot of different sights covering it.