Voltage detected across Micro USB cable shielding
If this is a LS device, you can't use LS-grade cables, the cable must be "captive". And there should be no LS detachable cables without shield.

If this is a FS device, then any certified cable should provide enough conductivity to keep voltage drop within 0.25V typical, shield or no shield (125mV over ground wire, and 125 mV over VBUS) with 500 mA current. If your device consumes more than that or you are using bootleg uncertified cables, it is not a USB problem.

Alternatively, there could be bad ground pin, broken, folded inside the cable itself, of cold solder on PCB.