All I can say is I'm not sure with the info I have. It could be that there is an air pocket trapped and when you turn on the pump, the air pocket gets pressurized and reduces in volume. Then when you turn the pump off, the pressure drops again, the air pocket expands and pushes the water back into the radiator.The engine and radiator are both in OEM location. Engine is not high. 4 gallons in and level still goes to bottom of tank when turn electric pump on, with no thermostat or restrictor plate. Comes back up to top of tank when turn off pump. Radiator is wide albumin 2 row. Just curious.
I'd run the pump, fill the radiator, then put the radiator cap on. There would still be air trapped but when the engine heats up and the pump is running it should purge the air out into the radiator and when the engine gets hot enough the system pressure will force the air out into the reservoir.