I've learned this once talking to one of the shoppers. I never tipped again after that / I tip in cash for the good ones once they deliver at my door.
From conversations with the shoppers, it also varies by store chain. Different chains have different deals with Instacart as to how the shoppers can perform their jobs, including if it's the same person delivering.
My impression is that when they get what they want, Instacart has shoppers that stay in the store shopping and drivers that deliver.