They might still have mobility issues. If you don’t want to cut the front of the litterbox open to make it lower and bigger, than maybe get a piece of wood or a brick or something else to act like a step to make it easier for them to get into the box.
Also, I assume the litterbox isn’t just the bottom. Those are usually terrible. I am guessing that there is a top/cover to it, right? They could be pooping in the box near the edge and it just falls out if there is no cover.
(something like this: https://www.petco.com/shop/en/petcostore/product/so-phresh-geometric-covered-cat-litter-box)
If this continues with a new litterbox with a cover, then you should consider bringing them to a vet. Cats are very clean animals so if they are pooping outside the box, it is usually for a good reason.