The fan available and licensed props and sculpts never position the neck brace correctly in the mask - it should sit further in, basically tucking inside the sides of the rear of the face mask. Most always line the edge of the neck brace with the rear edge of the helmet, making the neck brace stick out way too far in the rear, making it impossible to get the correct dome position. Also, most do not cast the domes thin enough, which means the crown is also hitting the inside of the dome and making it sit higher in the rear - this is also caused by some putting the disk on top of the head with all the greeblies on wrong - too raised in the back.
However, the original RotJ dome didn't sit properly on the reveal mask either, but they got away with it by showing another helmet just prior to Luke taking off the dome, so you never saw how weird the dome position really was.
The ESB reveal also has the separation line between the upper and lower part of the face mask cut different - following along above the upper tusk tube and then bends at the top rear of the tube and goes straight back to the back edge of the face mask - whereas the RotJ reveal cuts across the upper tube at the cheeks and follows the lower tusk tube to the rear of the mask.