Hello: I’d like to explore grounding/bonding a bit. You said there is an earth ground, but how is that done? Is there a ground line run back to the main panel ground, which is connected to a driven grounding rod? If the heater has it’s own grounding rod, it could be creating some trouble. A test is to see if there is any voltage at all between the tank itself and a ground that you know is tied into the main panel ground. You should test both with the heater running and off. Also is there a bonding wire jumpering between hot and cold plumbing above the heater?
Another suspect would be the water, but yours sounds good. Have your neighbors had a similar problem with heaters? That info would tell you if it’s a more general condition or something in your house.