I was leaving my pumpkin farm produced for a period on my computer, that is, I allowed the days inside Minecraft to pass so I had items to trade with the villagers. The problem is that I put a cross on the top of my house and let the blocks oxidize: this took days in the game. On top of the cross is a lightning rod, could it be the cause of the problem? Was there something you could do to make the blocks turn green forever?
The lightning rod was indeed the problem. Lighting will cause copper to be restored. You can wax the blocks with honeycomb to prevent this from happening.