This typically happens when the front to rear wiring harness has a damaged internal wire or wires. The front to rear harness runs from the front control console, down through the tiller (T part) through the front wheel frame and angle adjustment collar, and through the bottom frame to the controller. There is a wear area of the wiring at the front bearing of the steering tiller. There are 8 or more wires in a covered jacket that run through this area. Over time they rub and get pinched or kinked and broken. The test for bad wiring is exactly what you describe. Usually the broken internal wiring cannot be seen.
The best solution is to replace the entire front to rear harness with the new connectors at each end. The harness is relatively inexpensive and labor should be about 1 hour. Hope this helps.