How to measure shunt drop with an Arduino
You can't just stick one of the Arduino's ground connections wherever you want it for a measurement; the Arduino's ground is already set by its connection to a power supply.
Doing this would connect two points with different voltages.
Try moving the shunt to the south of the load, between load and ground, and keep the Arduino's ground at the same level as V1's ground.
If you can't do that, and/or if you want to avoid having to amplify the measured voltage and/or insist on measuring on the high side, you could consider using an INA260 or similar. Breakouts are available.