CMOS inverter in series
Clearly, your knowledge here about the propagation delay is sound, and it is the fault of the simulation designers or the settings for your simulation.
 Depending on which software you use for simulations, there will be big gaps like the one here, or smaller and less obvious ones. In the end, your knowledge, understand, keen observation and real-world testing and measurements are the best assurance that your circuit works as intended or expected. Whichever software you use, I would not rely on it 100%.