Without knowing the circuit and it's operation point details, I won't say it's a bug. It may be normal operation by design of the circuit.
I previously misunderstood suddenly as a sudden change over frequency. But the frequency characteristic of each parametric plot is pretty plausible. The only question is, why the transfer characteristic changes "suddenly" over operation point. If you imagine a transistor switch in the circuit, it's quite reasonable. So you have to analyze the signal path to see, where the sudden change happens. Besides a stage "switching" on and off it can be also a case of signal superimposition from parallel pathes, that cancel out.
Monte Carlo analysis requires calculating a new operation point for each iteration in my opinion, and the tools I'm working with are doing it. But depending on the circuit, the different operation point may be just the problem.