To be fair here: Poland did not try to limit the amount of grain for higher prices. Quite the opposite: they directed the farmers to increase output. And just when they did as they were told, Polands leadership realized that they are a too corrupt pile of shit to follow regulations. And so instead of being only transported through the country Ukrainian grain (that in no way is fullfillig EU requirements and thus shouldn't even be allowed to be sold in Poland in the first place) is flooding the country and crashing the prize.
The farmers being angry is natural because their prize is of course reasonable (they produce to much higher standards to meet EU guidelines which costs money).
The joke here is the government who could either start prosecuting the corrupt guys at the top causing the illegal dumping of Ukraine grain into markets where existing standards should automatically restrict it... or they can ban Ukraine grain to pretend to protect their poor farmers (from their own corruption cough - oh, sorry. I meant from the evil EU trying to kill their market obviously...). And there are elections next month and that government is running on pure populism, propaganda and anti-EU/anti-Germany sentiments anyway (which for them is same as the EU is -as we all know- puppeteered by Germany in their attempt to build their 4th Reich - I'm paraphrasing actual comments from Polish officials here...). So it's to nobodys surprise which option they chose.
PS: As should have been obvious from my comment: No, it's never actually "Poland and the EU". It's always poor Poland vs. the EU. Polands biggest government party has exactly that one topic and needs a lot of mental gymnastics for their "of course we want to stay in the EU but the EU is trying to destroy our country at every step" rhetoric to kind of work...