I genuinely think it’s just because their stomachs are getting used to digesting food, I don’t think it would matter what they are on x
My almost 3 week old had exactly the same issue when she was a week old, she is on kendamil too, I think they just need some time to adjust because they're so new to the world, I'd give it another week, we helped our little one to poop by rubbing her belly and cycling her legs till after a few days she started to poo and now she's no longer constipated x
We were on kendamill at first as well, my LB is nearly 4 weeks now and we switched him onto Aptamil comfort after the first week and he’s seemed much better since then x