It is not the explanation I learned when I was in Poland.

Officially, no one was buying the Ukrainian grain that was in transit in Poland. But some corrupt local officials did because it was cheaper. However, in their greed, they did not check why it was cheaper. It was cheaper because it was a commercial grade, not suitable for many food products. So, for that grain, there were few buyers. But, the grain from local producers did not get into warehouses.

To make things worse, most of the corrupt officials involved were from the ruling party PiS (Law and Justice), but the election was coming, which the PiS lost. Smelling it coming, PiS used its popular support in the countryside and dug out old nationalistic sentiments, some of them anti-Ukrainian.

This way, they shifted the faults of the grain market disruption from their own corrupt operatives to the Ukrainians.

I have a text on Medium providing more information, "What happened in Poland?"