My December figures are showing the setpoint reduction from 21 to 19C occuring at the scheduled time but the second reduction to 17C often happened around 50 minutes ahead of schedule.I'm not an expert, only had Wiser for a few weeks, so I am still investigating how eco mode actually works. I think eco mode switches the heating off early if it deduces that the setpoint temperature will be maintained until the next setpoint. To do this Wiser needs to understand how quickly the room/house cools down and what the outside temperature is. I could be wrong!
I suspect your deduction was correct. I've now simplied the schedule and switch from 21C to 17C at 22:30. I'll see what happens over the next few weeks.
