Current Regulation Drift
Copper has a temperature coefficient of 0.4% per degree Celcius. One square of copper foil, if the standard 1.4 mils (35 um) thick for 1 ounce/foot^2 foil, has 0.00050 ohms per square of area, for any size square. 

And copper has thermal-timeconstant of 9,600 seconds per cubic meter, 96 seconds per 0.1 meter cube, 0.96 seconds per 1 cm^3 (and also for any 1 cm^2 region of foil where the heat only flows along the long axis of the foil. And 9.6 ms TAU for 1 mm squares.

You have a copper-heating issue.

Draw out your PCB carefully and examine how the heat flows.

Are you using 4-wire Kelvin-sensing?