My dog is not eating

Diva is a 6 months old corgi. When we got her, she used to eat Real Nature Wilderness Wild Valley. It made her poop black, stone hard and smelling horrible. We changed her food gradually to another food (Wolfsblut Wild Duck) and she absolutely loved it in the beginning. 3 weeks in and she stopped eating her food unless she's starving and with a weird behavior: she takes the dry food in her mouth puts it on her bed and starts playing with it and then eats it. I have never seen her do it before that. She also used to eat her dry food if you throw it to her piece by piece but not in a bowl. I tried mixing her dry food with wet food and she devoured it. I also took her to the vet and asked the vet and told me that she does not have any health issues and I told her about mixing dry and wet food and she told me it's fine as long as she eats. In the last couple of days, I noticed the same behavior again: smelling her food and backing away and then coming back and eat it. Today she did not eat her food at all, I also tried giving her the wet food alone but still she did not eat it. She eats treats just fine and anything I give her like Cucumbers and apples... I am starting to go crazy on finding the issue, does anyone know of this behavior or did anyone go through with it with their pups?