I don’t know if it did. Any cable with a shield has to have the shield connected to the reference ground for the system and the shield is more to protect the center conductors from common mode interference than to prevent RFI or EMI radiation from the cable. I didn’t look up the BLS257 to see if it’s a brushless servo but it probably is. Which means you have a little inverter on the circuit board in the servo that produces the square sine AC power to drive the brushless motor in it. And you have a potentiometer that sends the servo controller the position of the servo output shaft.
Excessive “noise” can come from either the pot or the inverter if it has a failed cap on the driver. There is some magnetic field around those servos, created by the mag field around stator in the brushless motor. But it is very small and weak and should not really affect the compass much even if the compass is right by it - at least nothing that a compass cal with the servos operating can’t adjust for.
It is likely that servo has a problem, especially if it seems “jumpy” or the tail is excessively “twitchy”, I’m guessing a dirty pot.