IIR Filter Design Issues : MATLAB
This would appear to be a bug in the freqz implementation. I didn't dig enough to figure out why but even this doesn't work:

freqz(SOSb(1:2,:));


Even though angle(freqz(SOSb(1:2,:))) returns a legitimate result.

But I found this as a workaround:

[h,f,s]=freqz(SOSb);
freqzplot(h,f,s);


Unfortunately freqzplot is being obsoleted so you might have to resort to:

plot(angle(freqz(SOSb)));