I delivered food many many man…you get the idea, years ago, long before the delivery apps, and even before the interwebs. The short answer is: No, we/they are not supposed to do that.
I cannot be sure, as I quit because of asshats who cheat the system and screw over the customer. I understand why they do it, we stopped making money once the apps came into existence. Back in the day, I made $300+ after expenses on Valentine’s Day, and now it be a good night if I made $100 before expenses. However the jerks will run multiple phones, accounts, and apps, at once. So they will pick up your order, head to McD’s to wait for the order to be ready. Stop to deliver it, and if there is another pickup on the way to your house, guess what, they are stopping. They call you because it can/will/might save them a minute or two, added up over the night, that could amount to a couple extra deliveries.
Again, I’m only assuming by what I’ve seen on the other side of the app and the asshat drivers who have taken over, and I do NOT condone that practice at all.