Always at the first floor is that the bottom floor? I did assume it’s a hydraulic pump elevator.
Does the building have a cleaning crew that arrives after hours? If so, might they be blocking the doors open to mop and dry the cabin floor? That could be an issue. Just a thought.
I Would also examine of the workings of the hydraulic valve, and checking the timer that shuts the pump off if it runs too long and does not reach an intended destination (or just releveling up to floor level, which is suspect in your case) which is reset by cycling the power. Trouble is, most shutdown faults are reset by cycling the main power, but yours is narrowed down to after hours.
