certain pumpkins unable to be traded

after setting up an automated pumpkin farm, i found that several stacks of pumpkins were incompatible with farmer trades. upon further investigation, i found that placing and mining the pumpkins while facing north fixes them. the main problem is that a single "wrong" pumpkin will mix into a normal pumpkin stack and poison it, making all 64 pumpkins untradable until they are placed and mined while facing north.