I'm using an Arduino and Mosfets to modulate LED RGB light strips. The INA219 works fine at full voltage but as it is lowered the mA values get more and more erratic.
That is because the modulation is done with PWM. You are not really lowering the voltage, just sending shorter and shorter pulses of the same voltage. The corresponding current draw will not be a constant value. Depending on how you have things connected, you might be able to smooth the signal a bit with a capacitor. Or you can filter it in software by taking the average of multiple readings.