There have always been a lot of Poles living in Germany, who keep Polish names, but who are thoroughly German.
You wrote:
“There have always been a lot of Poles living in Germany, who keep Polish names, but who are thoroughly German.”
That has been the case only since WWII. But since the 15th and 16th centuries there have been Germans with Polish sounding last names, however. You also used to find Old Prussians with Polish and German names. That was the case from the 13th century onward - especially among the junkers class of Prussia.