I wrote a thesis on policies with regard to minorities, which included times before II World War (unfortunately it's in Polish but if you know the language then I can send it to you). Germans were mistreated in multiple different voivodeships. They were blamed for the I World War and the years before the I World War when Poland did not exist. Their belonging were taken from them and they were stripped of many civil laws. I wish I had some sources in English but unfortunately all I have is in Polish. However, there were no need for propaganda because there was a widespread mistreatment of the German minority in Poland.

EDIT I: I recommend reading about Polish voivode Michał Grażyński, who was in charge of the Silesian Voivodeship before the II World War and his policy was basically about mistreating Germans.

EDIT II: Browarek Tomasz, Henryk Chałupczak Mniejszości narodowe w Polsce 1918-1995,

Wydawnictwo Uniwersytetu Marii Curie-Skłodowskiej, Lublin 1998. This is the book which describes the Polish policies regarding minorities including the German minority. The part about the German minority before the II World War is written on the pages 274-277. Unfortunately the book is in Polish but I'm sure that there are references in other languages or maybe someone quoted this book in some paper written in English. I will upload a piece of my paper and it's translation later today and if someone is interested in reading my thesis on policies regarding minorities in USSR 1917-1956 and Poland 1918-1956 then please pm me but keep in mind that the paper is in Polish.