If you are setting the temperature at 20 by twirling the dial or pushing up/down arrow keys, that's only a temporary instruction to the Nest. It will revert to its underlying schedule at the next set point in that schedule. If the schedule calls for 23 at the time you see the Nest change to 23, that may be what is going on. (I apologize if I don't understand your description of the problem; I'm not sure what you mean by "manual".)
Ok, I'll try setting a schedule in the app. on my phone.
I have set 19 degrees at 16:00hrs. every day of the week.
What does it do for the other 23hrs of the day if 16:00 is set to 19 degrees?
I guess (hope) it just stops at 19deg until any change in the next scheduled event?
Is there a manual or something explaining all this somewhere?
We haven't heard from you in a while so we'll be locking this thread if there is no update within the day. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the Community.