DIAGNOSIS This is usually caused by a slight gap or moisture barrier between the touchpad membrane and the control board, often due to a loose connection, or less commonly, a failing membrane itself.
FIX First, try a soft reset: unplug the microwave for 5 minutes, then plug it back in. If that doesn't help, the internal ribbon cable connection might be loose. To check, unplug the unit, remove the control panel cover (usually held by screws behind the grease filter and along the top front edge). You'll see a flat, wide ribbon cable connecting the touchpad to the main board. Carefully disconnect and reconnect this cable to ensure a solid connection.
IF THAT DIDN'T WORK If reseating the cable doesn't fix it, the touchpad membrane itself is likely defective and needs to be replaced as an assembly. This part is often integrated with the entire front control panel.
HEADS UP Be extremely gentle with the ribbon cable and its ZIF (Zero Insertion Force) connector latch. They are fragile. Use your fingernail to flip the latch up to release, and slide the cable out. Don't pull on the cable directly.