Pilot reported propeller control malfunction caused the left engine to go into reverse on touch down; causing the aircraft to depart the runway; striking a runway edge light.

Narrative

Here are the events leading up to our runway excursion at ZZZ. Name (Pilot Monitoring) and I (Pilot Flying) were coming in for landing at ZZZ on Runway XXL. The approach was a visual to [Runway] XXL. We turned on course about 5 miles out and it was a stabilized approach from there. Speed was at Vref reaching the runway. We did float for longer than normal; because of higher than usual idle torque. In the flare; I (the Pilot Flying) went to ensure the power levers were at idle. As I pulled back to ensure they were at the gate; the left engine commanded reverse. My immediate reaction was to push the left throttle out of reverse as the wheels were touching down and attempted to straighten the nose and level bank with differential power in addition to rudder and slight aileron input. Then I brought both throttles into idle then eventually reverse. The Pilot Monitoring got on the controls somewhere during the flare and applied rudder and then differential braking while aiding with throttle management. Our initial attempts to straighten out the aircraft led to it veering right and we were unable to straighten out the aircraft before it departed the runway to the right side. We believe we struck a runway edge light with the number 2 Prop in the process. Once at a full stop we shut down without further incident. No other damage was observed. No injuries were sustained. We became aware as it happened. As soon as the left engine commanded reverse and the airplane started to yaw we reacted. In reflection there are a number of factors that could be contributory. The left engine went into reverse either because of a mechanical failure of the gate to stop it; or sticky triggers; or because I lifted the throttle above the idle gate. We could have gone around; but the initial attitude of the plane did not suggest to me that was a good idea. Because it happened so fast there was no strong transfer of controls; instead we both made inputs. Our long duty and flight times for the day likely increased fatigue hampering reaction time and ability. Our reaction was to counteract the forces applied to the plane by the left engine going into reverse. We applied aileron and rudder input. Additionally I applied differential power by taking the left engine out of reverse and forward slightly. Then we attempted to keep the plane on the runway using rudder; differential braking; and reverse thrust. Careful attention to flight idle gate; and reduced pressure on the throttles. Clearer communication in a transfer of controls. Possibly the use of the nose wheel steering at the appropriate time/speed or a go around after the initial recovery attempt.