For most of their life, your child has pooped standing up or squatting, wrapped in a diaper, often tucked away somewhere quiet. It is familiar and it feels safe. Peeing on the potty is quick. Pooping takes longer and needs the body to relax and let go.
Now they are asked to do it sitting, with their bottom over an open space, a strange echo, sometimes an adult watching and waiting. The sensation of poop leaving without a diaper there can feel odd or even a little alarming. Some children feel that their poop belongs to them and do not like watching it disappear.
So they find a workaround: asking for a pull-up, waiting for nap time, or slipping behind the couch. These are sensible choices from a small child's point of view. They show your child knows when a poop is coming, which is exactly the skill you need.
The risk to watch for is holding. If a child starts holding poop in to avoid the potty, poops can get hard and painful, and the problem changes shape. That is why keeping poops regular matters more than where they land, for now.