Abnormal voltage drop across n-mosfet in multisim
You have a 16 ohm resistor in series with the source and as soon as the MOSFET starts conducting that resistor develops a volt drop that degrades the effective voltage applied between gate and source and leads to a situation where the MOSFET operates no longer as a switch but linearly. It's called source degradation.

correct me if im wrong but i would of expected something drastically
less with a RdsMax of 1.5mOlms

The MOSFET's on resistance is no longer relevant with a circuit that produces source degradation. If you want to switch the full 50 volt supply across the 16 ohm resistor, the you need to connect the source to 0 volts and put the load in series with the drain.