Induction Motor Switching Surge
You should be able to measure high DC resistance from winding to case and earth ground the case.

If your 0Vdc is floating in your electronics this may require a 1~10nF cap from 0Vdc to earth ground as well.

The voltage created is from the 20mA coil current and mutual inductance .  

An alternative solution without using earth ground is to add an RC snubber but this must be computed based on the motor inductance and frequency with possibly &lt;20 mA AC using >=10kohm 5W and some C value such that |Xc|=|XL|

But it is possible  your measured voltage could be a measurement error since your choice of reference is the scope’s long inductive probe earth ground (if 20MHz ringing pulse)  not the electronics 0V which may be floating or earth grounded. Remember, ground is just where you define 0V.

Easiest possibility

If the transient is crosstalk from the motor coil switch arc voltage at turn off, this can can be snubbed with a film cap and series R across the switch and using twisted pair or STP cable.