Let's consider for simplicity Middle-Square Weyl Sequence RNG:
We can choose there parameter for independent stream generation, we can call it key. Let's consider we would use it as a stream cipher, so stream is xored with plaintext.
But what it mean to broke such a cipher? Even if we hack the key, we can't compute the seed in invertible way, beacuse regardless of the fact that minimal period of that PRNG is guaranteed, there is non-invertible transformation. We are loosing some bits after every step.
In that case it is not enough to find a key. When we would say that this cipher is broken? Does there have to be a seed recovery, to conclude that cipher is broken? Or maybe we could be talking about varying degrees of breaking such cipher? Because on one hand if we break the key, we can usually generate next bits of the stream. But so what if we can't read the strem backwards until we get the seed back.
What is more important in such a case for the victim, that the attacker does not learn the key or the seed?