Hello Coppice,
Maybe there will be high potential difference between the neutral point & the 0V output of the AC/DC converter considering the voltage at the neutral point is 220VAC with respect to the line voltage.
The difference between the neutral voltage and the 0V voltage will be small, being only the drop through the wiring - only millivolts, even with a high current load. Even if the live and neutral connections were reversed, the isolated supply would mean this is not an issue. If you use J5 to disconnect the EVM's build in modular power supply, and use an external power source, you need to take great care about isolation.
The main thing is when the line voltage gets into the MCU, the line voltage should be in reference to the ground point, so there should be connection between the neutral and the ground.
The incoming neutral connects directly to AGND, which in turn connects to DGND. Since the ADCs are fully differential there is no particular reason why the line voltage needs to reference to the ground point. The difference between them just needs to be kept within the range which the ADC can handle. In this EVM design the line voltage will, in fact, be measured with reference to ground, since the incoming neutral is tied directly to the MCU's grounds.