TL;DR of the TL;DR of the TL;DR - Tracking disease is complicated
TL;DR of the TL;DR - Plague basically hit everywhere, the "spared" region in Eastern Europe is more complicated geographically than just Poland, and more complicated than present/not present.
TL;DR - Southern Poland and the Kingdom of Bohemia were significantly less devastated by the plague, but it still roughly decimated the region in 1350. The difference between 10% and 30-50% is huge, but to say that the plague wasn't in Poland doesn't hold up to scrutiny. My answer looks at the mapping of the plague in the region, and the difficulties and theories that have gone into that ongoing debate.
It's important to note that there isn't quite a consensus that all of Poland weathered the Black Death. In his article "A Plague on Bohemia? Mapping the Black Death" in Past and Present, David Mengel discusses the various ways that historians of the plague have tried to illustrate its effects in Europe, particularly in Bohemia and Southern Poland, two big examples of regions that were largely unaffected by the plague. Mengel briefly mentions other similarly spared regions, such as the Low-Countries and some of Basque country, but Southern Poland/Bohemia remain his main point.
What he concludes is that its impossible to illustrate the incredibly diverse responses to the plague throughout these spared regions in something like a map. An example of a map he critiques as too broad can be seen here. The shaded region over Bohemia/Southern Poland represents the places largely spared by the plague, although Bohemia had roughly 10% of their population die in the 1347-1350 period commonly associated with the bubonic plague. For Mengel that 10% casualty rate isn't attributable to any one cause, there are too many diverse reactions and experiences with the plague throughout this region to accurately reflect them all in one map, or one argument.
As much as its important to realize that its more nuanced than Poland being entirely spared in the 1347-1350 plague, that's not really the answer you were looking for I'd imagine. Mengel also surveys some of the local scholarship on the topic. Some theories which cite one cause for the sparing of this geographic region include: 1. Distance from the coastal trade routes and the relationship between maritime trade and the spread of the plague, 2. the absence of plague-spreading animals in the eco-system, 3. ethnic immunities, or 4. mountainous terrain contributing to added isolation.
Aside from the ethnic immunity argument, which doesn't hold up when you consider that this same geographic region was devastated by a different plague outbreak in 1380, all or none of these could apply to the disparate regions and zones that make up the 'spared' region of Bohemia/Southern Poland. I've used Southern Poland because a common point in illustrating the spread and path of the plague is to emphasize the importance of coastal regions and port cities, which were devastated by the outbreak throughout Europe.
