DIAGNOSIS The Auto-Schedule and/or Home/Away Assist features are overriding manual adjustments. These features require consistent manual input to learn.
FIX First, disable temporary manual overrides. Use the schedule. Go to Settings > Auto-Schedule. Turn it OFF. Then, go to Settings > Home/Away Assist. Turn 'Use phone location' OFF. Set 'Use thermostat sensors' to 'Use sensors for temperature control only'. Now set your desired schedule manually in the Schedule menu.
IF THAT DIDN'T WORK The Nest may be detecting 'Away' due to lack of motion. Check the 'Activity' display on the thermostat history. If it shows long gaps, the motion sensor may be obscured. Ensure it has a clear view of the room.
HEADS UP With Auto-Schedule off, the thermostat will not create or modify your schedule. All temperature changes must be programmed manually or via the app.