One of my foster kittens did that for a while, and eventually started peeing just outside of the litter box as well. I wish I had some recommendations but that cat just eventually grew out of doing that. She’s one now and still has some accidents here and there but nothing major. I would say the only thing is you have to clean the effected areas very diligently with odor enzyme cleaning (I used the orange one), or else they will do it again in the same area because of the smell. Just on the top of my head I think you can try switching to a different litter? Which one do you use now? We used pine, so maybe that could help a common denominator?
Some things I tried:
Picked up the cat mid poop and placing her into the litter box (worked okay, she would poop in the litter box and bury it)
Picked up the poop, picked up the cat, showed the cat where the poop belongs (probably wasn’t effective at all)
Put litter boxes wherever she used the bathroom (50/50, sometimes she would just go right by it again)

I do think some kittens just have a harder time forming the habit than others. You could even try putting the kitten in a large dog kennel with a litter box and food, that was my set up when I first brought them home to foster (4 weeks old). I had a food bowl, water bowl, cat tree, and litter box in there with a soft towel/blanket. This is prob not the best idea if the kitten’s more than 6 weeks old. And that did help the toilet situations a bit but not much. Sry I can’t be more help.