Not exactly mentioned so far, but it is generally the case that a power supply which has a ground lead on the AC cord, will also have that ground lead connected to the negative of the output. Which is why Mike is suggesting that it has something to do with the odd behaviour.You can verify that connection with a multimeter.
for what it worth here the results of those circuit.
Yes with a power supply with that much current output you are likely to have a 3 terminal mains connector. However in my experience the outputs still float rather than being connected to ground.