This appears to be two different issues. A heating element is only a wire with a certain amount of resistance, inside of a sealed tube. It is designed to get hot enough to heat the water surrounding it in a reasonable time. The amount of heat produced is a function of the voltage applied and the resistance of the element. If the voltage is lower or higher than 240 VAC it will take more or less time (respectively) to heat the water, but will otherwise have no effect. The same goes for an incandescent light bulb. It is purely a resistance that gives off heat and light, with less voltage giving less light. A motor, on the other hand, uses the alternating current to provide an electromagnetic field to rotate the shaft of the motor. That motor is engineered to meet certain physical requirements, such as moving something. If the voltage is more than roughly 5% lower than it should be, the motor will strain and will start to heat up, shortening the life of the motor.
I agree with House Doc. The first incident is likely from a defect in the heating element. If there was a crack in the element's tube, it could allow water to leak in and produce steam.
The second incident is with what I believe the OP is describing as the thermostat tripping off. The thermostat trips when the water tank gets too hot (~180 degF). That could be a faulty thermostat or a shorted heating element where the resistance wire is shorted to the metal tube, which is effectively connecting it to ground. Since only one side of the element is switched by the thermostat, the other is still providing 120 VAC and the short provides the completed circuit. The element will continue to heat the water until the thermal fuse trips. Both of these scenarios are very unlikely with a new water heater, especially with a new element, although the shorted element is more likely, since it took several days for the thermal fuse to trip.
A circuit breaker is essentially a thermal fuse. It contains a bimetal strip that heats up as more current flows through it. They are designed to bend far enough at a certain current flow that the metal pulls away from a contact to open the circuit. They are not precise. A voltage surge will allow more current to flow through a heating element, but it has to be a sustained higher voltage and it must be high enough to trip the breaker. Most elements are 4500 watts, which at 240 volts equals a current draw of 18.75 amps. The NEC requires that the circuit protection be rated at 150% of that, or 28 amps, which would be a 30 amp breaker. IF one of the strands of wire were just touching the metal tank, it might allow enough current to allow the element to heat at 120 volts. If the stranded wire were clearly touching the tank, it would likely trip the circuit breaker.
As with Breplum, I don't buy from the big box stores, and if I have a problem with a product, I take it back to my dealer.