LM2596 bypasses resistor
Switching regulators like this operate as a closed-loop controller and this means there can be a susceptibility to self-oscillation if the feedback loop is meddled with. Opening the loop at the PCB pot and extending the pot wiring several inches is probably enough to cause all sorts of problems. It is likely that the closed-loop control has become unstable and you get strange results depending on where the wired external pot is positioned relative to the PCB.