Slouching Baby

I have a 10 month old baby who I think is intentionally slouching in his high chair. I will sit him up straight and he will stay up straight for 1 second and then push his pelvis forward to be more slumped. He also does this in his stroller and sometimes leans to one side. He sits up fairly straight when he is just sitting on the ground. I know he doesn’t like any form of being strapped in. I keep pulling him up straight and he goes right back. I think he does have the strength he just doesn’t want to do it in the stroller or high chair? Not sure how to fix this?