the argument that Poland was resistant to the plague comes from a lack of statistics attributed to Communist rule in Poland during the 20th century.
evidence against Polish omission from the plague includes societal changes often associated with the plague. In particular, during the second half of the fourteenth century, changes included the increase in the size of Polish forests, the increase in wages in Kraków, and the collapse of grain prices. These societal changes are widely attributed to the demographic shifts resulting from the Black Death and were documented in both Poland and other European countries.

So, basically, a successful re-writing of history.

There are some suggestions that Poland was less hard-hit by the Black Death