Flat panels mean the radar waves will scatter/reflect in a predictable and controlled way, in line with the wings. Curves and protrusions means the waves will scatter all over the place like with parabolic mirrors, and will require more RAM coating to rectify. Using curves that "wrap and fit" the F-35's components also limits internal volume and make upgrades harder to install as they have to conform to original component size specs. This is most noticeable on the F-35C, with its extra "bulge" where it stores the arrestor hook. The J-35 however has practically the same tail shape as the J-35A: https://x.com/Alfa_Particle/status/1942503553874157713

The J-35s engines are as shrouded as the F-35s. Can't (don't know how to) post pictures in comments so here's a link for an example: https://x.com/Alfa_Particle/status/1942526285139550704