Apparent bug in comm.PhaseNoise?

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I'm trying to add phase noise to a signal, using the examples provided, but it only works part of the time. Is there some strange limitation on which values of phase noise are allowed?

clear all
nbits = 1000;
data = randi([0 1], nbits, 1);
modData = pskmod(data,2);
phNzLevel = [-70 -80]; % in dBc/Hz
phNzFreqOff = [2000 20000]; % in Hz
pnoise = comm.PhaseNoise('Level', phNzLevel, 'FrequencyOffset', phNzFreqOff);
y = pnoise(modData);
scatterplot(y);

The code above works. But if I change line 5 to the following, then no phase noise is added to the signal. I've increased the noise, so it should look worse than before. Why doesn't it work with these values?

phNzLevel = [-50 -80]