Achieving long term stability of a MOSFET current sink
As ambient temperature varies, the LED voltages vary, the voltage across the FET varies, thus the heat varies and the  heat to dump varies, thus the resistance of the sensor resistor may vary.
You need an error analysis to model these variations.
==============================================
You can place the resistor in a region of the PCB that has little thermal linkage with the FET.
Standard copper foil has 70 degree C per watt per SQUARE OF FOIL thermal resistance. And with only 20mA to the resistor, you can use LONG THIN races from the FET to the resistor. Under that trace, have GROUND PLANE so the heat gets mostly shunted away and does not reach the resistor.