Full-wave rectified signal issue
This is generally caused by capacitance in the load and/or measuring device, or stray capacitance somewhere in the circuit. A high resistance load may not be able to discharge this capacitance fast enough to drop the voltage to zero during the zero crossing period. 

In your case the effect is only occurring on every second half-cycle. This indicates an imbalance in the rectifier circuit, and suggests stray capacitance between the generator and ground may be the culprit. 

I simulated your circuit in LTspice, first with the generator fully isolated and then with 15nF between its local ground and circuit ground.

The circuit:-



Without stray capacitance:-



With stray capacitance:-