Sample YF116731 on YFull is my maternal grandfather's brother, Władysław Meller. He is from Wielkopolska region.
Despite having a Slavic haplogroup he now does have a German Y-DNA match with TMRCA around 650 years ago:
https://www.yfull.com/tree/R-Y54572/
https://i.imgur.com/DIyEqJd.png
So it is possible that some Germanized Slav became a Meller (Germanic surname) and then migrated to Poland.

