Answer: What you describe does sound peculiar. If the plant looks otherwise healthy, I'd uncurl one of the new leaves and check for aphids. When they feed they can cause curling and distortion of leaves with eventual yellowing and then browning. If you find aphids, use Insecticidal Soap to control them. If your plants have many leaves already, you might want to just cut off the affected leaves. If your plants are just starting to put out leaves, hose the pests off the leaves or use Insecticial Soap to stop the feeding and save the leaves.
Best wishes with your pumpkin plants.
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