you can just take a look at the rear of F-35 vs F-22 vs J-20. You will see how much F-22 hides its nozzles. F-35 would be worse than F-22 from the rear. Between F-35 and J-20, you can see a lot of gaps at the back which could also cause greater radar deflection.
F-35
now J-20
just more gaps possible deflection issues for J-20 at the back. And that's even assuming the canards trailing sawtooth patterns are scattering the radar coming off the delta wings rather than deflecting them back. I think there could potentially be issues there too.