It seems like the expression pedal input on your Stereo Polyphase pedal is behaving strangely, only working with Control Voltages from devices that aren’t grounded via the earth pin of their mains plugs. This could be due to a possible issue with the socket’s ground connection. It’s likely that the socket may be missing its ground connection or has one when it shouldn’t. I recommend reaching out to the technician who repaired your pedal to have them check the ground connection on the expression pedal socket for proper configuration.

Best of luck with your modular synth system!