I have a bunch of these on service. I assure you the issue is the sio board. To the previous reply you are correct, it happens at night and doesn't involve any inputs. Because of camera systems I have in place I have watched this anomaly happen. When one elevator gets called via Hall call it happens then for re homing to a lower floor, which is essentially a supervisory function, in this case between sio boards. For starters swap the sio boards between the 2 elevators and let me know what happens. Always only one bad one out of a duplex, triplex, whatever. Remedied my issues.