According to the Dessalator manual the system should shut down after one in this mode of operation. See the excerpt from the manual below.
Fresh water should be always used with no pressure when going through the system (pressure dial turned all the way anti clockwise) and the system should always run with no pressure after a fresh water flush to dump all the fresh water that are in it, before making freshwater from sea water (also with the pressure dial all the way anti clockwise). When running the water maker with the dial all the way anti clockwise, it will shut itself down automatically after 1 minute. Only then, the water maker is ready for use.
When introducing the sterilizer you are operating in this condition. I believe your system is operating correctly. You will then go through the flush cycle when you put the system back into service.
Brass Ring A54