Germany put out a document soon after the invasion was over detailing Polish atrocities. They showed many pictures of civilians killed brutally (bayonets, bludgeoning, bullets, etc) and in advanced states of decay (predating the invasion). They've also told several stories of civilians rounded up and death marched at the beginning of the war all the way to the east of the country. Finally, at least one British diplomat mentioned rough treatment of civilians prior to the war breaking out.
White paper: https://www.wintersonnenwende.com/scriptorium/english/archives/atrocities/pagmp000.html
Could the atrocities be fake? Sure. We've just got their word for it. Everyone in the west ignored it, and post war hundreds of thousands of books were burned by the Allies occupying Germany. Nearly every German that had been living in disputed territory was ethnically cleansed, exiled or killed. Even today it's illegal in Germany and Poland to produce apologia in any form for the 3rd Reich. Could they be real? I'd say there's a high chance.
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