Having used plain water for a time may have caused some growth of galvanic corrosion and scale inside the tubes of the radiator which has reduced the flow volume enough that now a point is reached (after 20 minutes) that the radiator can no longer cool the engine.
The presence of any amount of mineral or metallic scale deposits in the tubes reduces the flow below design parameters. Or put another way, reducing the coolant flow through a radiator is like having a smaller radiator.