You are rolling a ball along a knife edge expecting it not to fall off. You get lucky with transient, but with PSS the ball falls off. It should not be too much of a surprise. A comparator is designed to be very sensitive. Indeed you would never expect to see the transient results on a real circuit.

-Ken

You might think, because the circuit is perfectly symmetric, that the ball would just roll perfectly balanced along the knife edge. And indeed it does in transient. But the simulator introduces very tiny amounts of asymmetry, if for no other reason that the simulator is being run on a von Neuman machine, and so things must be done serially. For example, the nodes are placed one at a time into the matrix and then the corresponding rows are factored, so a node from one side of the circuit will be processed before its pair on the other side. This can inject small amounts of asymmetry on the order of machine precision (1 part in 10^15) into the calculations. This asymmetry can be seen by the comparator, which might cause it to latch one way or the other. The PSS does more computation than the transient, and perhaps is more sensitive to these small errors than transient.

The point is that in transient the ball is slowly rolling down the entire edge of the knife without falling off. That is a spectacularly unlikely thing to happen. You should not be asking why it happened to fall off during PSS, you should be asking how it could have possibly made it all the way across the knife without falling off in transient. The answer to that question ...

Luck.

-Ken