Some toddlers are afraid to poop on the potty because they are not really used to sitting on the toilet and flushing the toilet.
Sometimes it's the sound of the flushing toilet that scares the toddler when they are really young so sometimes you need allow the toddler to leave the bathroom after pooping on the toilet and then flush it until they get older.
But in those cases the toddler really isn't potty trained.
A toddler becomes fully potty trained really when they can go to the toilet or the potty chair unassisted and can pull their pants down and wipe their own butts and use the toilet without the parent or caregiver helping them.
It's usually best to wait until the child is around 3 before potty training although my mom potty trained me at 18 months old which I actually believe is too young.
Also some toddlers are afraid to poop on the potty because they don't want to lose a piece of them which is the poop and they are not used to the poop leaving their body and going into the toilet.
Instead the toddler has been used to pooping in the diaper which feels different than the poop going into the toilet instead of being mushed up against the toddlers butt.
Eventually the toddler will prefer to use the toilet instead of a diaper.