Two facts based on the principle of GAN could be taken to explain this observation: On one hand, a discriminator with too low capacity (such as "Filter[1]") is less capable in distinguishing human responses from generated ones, which will backpropagate inappropriate signals that misleads the generator.
if the capacity of the discriminator is too high (such as "Filter[1, 2, 3]"), in the adversarial training, the training of the discriminator will converge too fast before the generator being sufficiently trained (Durugkar et al., 2016).