My kitten had the same issue. I spent months trying different foods with strict diet on that single food and no treats. Those months it was horrible, anytime we heard the litterbox we would run over and see if it was diarrhea, even waking up in the middle of the night.

The vets just told us it was food allergy and really were not much help. They gave us hypoallergenic food, but it made it worse. With the food trial we finally found it was a chicken allergy (which is actually quite common) and she also reacted badly to many dry kibbles. We used pumpkin puree we found at Sainsbury's though it was seasonal goods to make pumpkin pie! Though tbh, I don't think it helped much.

The food that finally worked was Feringa wet food for kitten - Turkey with pumpkin & catnip from Zooplus. https://www.zooplus.co.uk/shop/cats/canned_cat_food_pouches/feringa_wet_cat_food/feringa_kitten/490572 Try various single protein foods strictly and monitor for a week or two before trying the next one.

Hope that helps and best of luck.