[GTP Dad]

Welcome to AF!!

It appears that the problem exists because the water forms a scale inside the radiator due to corrosion thus stopping the leak. I know someone will probably say that the radiator is plastic, even so the issue is related to the water forming scale that blocks the hole. When you introduce antifreeze it contains anti-rust and scaling properties that eventually open the hole back up and allow it to leak. I would repair or replace the radiator as you stand to lose a whole lot more than antifreeze if the block freezes this winter. Also with pure water you are risking problems with the water pump and a good bit of rust issues inside the block and head that could force you to do a teardown or replace the engine.