The current in the loop is i.
Voltages on the resistors are:
Voltages on the measurement points (vs.GND )are:
Vn1= VR2 = i*R2
Vn2= -VR3= -i*R3
It is not that "the signal is inverted",
It is just the way you measure and what is your reference point !
For instance, if you moved the reference point to be n2 itself ,
all voltages would measure the same "polarity".