Limiting current without dropping volts
It's not possible. You have a fixed voltage, 12V. You are attempting to enforce a current limit, 18A.

The trouble is, once you hit the set the current limit, if the load continues to try to draw more than that 18A limit, the only means left to keep the current down is to start dropping the output voltage.

In other words, any load resistance below 12V/18A = 0.66 ohm will begin to drag the voltage down. Think about it: taken to the extreme, if you had a dead short (load resistance = 0 ohm), any voltage above 0V would result in infinite current.