PSTB loop-gain Phase-plot : error at lower frequency

Hi.

I am simulating the common-mode loop gain of common-mode feedback loop of a fully differential amplifier, through PSS/PSTB analysis (using "diffstbprobe").

But, the phase-plot obtained shows a low-freq (dc) phase of 0° (instead of 180°) !

Please see the attached screenshot - the left half phase plot is as expected (obtained from another circuit), but the right half plot seems erroneous at low frequencies as I stated above!

I am using: PSS errpreset=conservative, and reltol=1e-5, vabstol=3e-8, iabstol=1e-13.

Can you please suggest a way to obtain the actual phase plot profile from the PSTB.

Thanks!