Welcome to Android Central! Strong magnetic fields can potentially interfere with normal touchscreen usage, as discussed in this thread about the old Blackberry Playbook:
Magnetic fields vs Touchscreen - BlackBerry Support Community Forums. I would wonder if the internal charging circuitry is generating an electromagnetic field that is not properly shielded, and is therefore causing issues with the touchscreen only while charging. I'm not an electrical engineer by trade, so I'm just piecing things together from the web.