Unexpectedly N-ch MOSFET high drain to source voltage in "on" state
Pay careful attention to the datasheet.

RdsOn = 9 mΩ max @ Vgs=5V  (6 mΩ @ Vgs=10V)

  Symbol      Parameter    Test Conditions             Min. Typ. Max. Unit
 1VGS(th) Gate Threshold Voltage VDS = VGS ID = 250 µA   1  1.7  2.5    V


Note the low current of threshold

As a Rule OF Thumb  to get good results;


use at least 2x Vgs(th)max


but often Vgs= 4x Vgs(th) Max or more 

Look for  a current rating of 10A application for low voltage
But most importantly, if you want low T rise in switch Use delta T=Rja=Pd ['C]


or choose % loss and then RdsOn= % of R load so for 5% choose 5% of 20m
Let's assume you used 5V Logic drive









simulate this circuit &ndash; Schematic created using CircuitLab

I suspect you must be using more than 5V as worst case is worse than yours.