Ethernet transformer saturation at low frequencies
This on-line simulator should help you understand. I've entered the capacitor and winding inductance value in order to see what voltage you can expect to see across the driven winding: -



So Vout above is representing the voltage across pins 15 and 16 of your transformer (no need to look at the secondary): -



As you can see, below about 8 MHz it acts as a 2nd order high pass filter and the signal level a hundred times below this frequency (about 80 kHz) is down by 80 dB. At 800 Hz it will be down by 160 dB and at about 80 Hz (near enough 60 Hz) it will be down by 180 dB.

If your hipot voltage is 500 V, then the voltage seen on pins 15 and 16 will be 0.5 uV RMS. Is your measurement equipment capable of seeing this?