Found a paper
Electromagnetic Scattering Characteristics of Outer Flaps to Axisymmetric Nozzle
It basically says that the gap between the outter layer petals (outer adjustment sheet) and the nozzle (the inner petals) increase rearward RCS.
A natural solution is to cover that gap as much as possible, that is the small caps on the engine nozles of J-35A, J-20 and F-35

J-20
F-35
So I want to say that so far the only visible difference between engines of J-35A and J-35 is EM signature reduction measure, there is no visible evidence to indicate that they are two different engines of different generations, in the same way as many different nozzles of WS-10 on J-20, J-10 and sinoflankers.
We know that both J-35 and A are in their early depolyment/integration phase. Navy's priority now and in the coming year is to get the aircraft land and launch from the carrier and train enough pilots, a small improvement to EM signature isn't so high priority before entering battle operation. They got few earlier variant of engines to work with and they do the job well.