Not familiar with this copyright and character, but I'm assuming it's an actual error and not a character who canonically has only four toes.

She also only has four fingers on each hand. It might be intentional.

I think it's the artist's style. I took a look through their pixiv gallery, and just about every character they draw has four fingers and toes.

That's interesting, it seems there's a mix from image to image, and what looks like a recurring set of pointy-eared girls with four fingers and toes. It does seem intentional now.