A toddler that refuses to poop on the potty but will pee on the potty is very common and eventually the toddler will go poop on the potty.
Usually a toddler will refuse to poop on the potty and hold it in or request a diaper to poop in because the toddler is either scared of letting their poop go into the potty or they just don't want to sit on the toilet waiting until the poop comes out.
Sometimes it's because the toddler is used to the poop going into a diaper and sitting there next to their skin and not pushing completely out into the toilet.
Going poop in a diaper and going poop in the toilet is a different feeling for them and they are used to pushing the poop out into the diaper which the toddler still feels is with them because it's still near their bottom.
When they poop in the toilet the poop goes into the water and is no longer touching their skin so they feel like they lost a part of themselves.
Sometimes it's just that a toddler is just too busy playing to want to sit on the potty to poop so sometimes they will just hold it in and then later poop in their pants or poop in their Pull Ups instead.
Or the toddler will want a diaper to poop in.
Honestly if you have a toddler that will not poop in the potty but will poop in a pull up or diaper then if the toddler is holding back the poop for too long then just give the toddler the diaper to poop in so that they do not get health issues with being constipated.
Eventually the child will use the toilet for both poop and pee and will no longer need the diaper to poop in.