DIAGNOSIS The Nest's built-in occupancy sensor isn't detecting you, or your phone's location isn't being used correctly for Home/Away.
FIX First, turn off all auto-scheduling features. Go to Settings > Auto-Schedule and turn it OFF. Then, go to Settings > Home/Away Assist. Turn off 'Use phone location'. Also turn off 'What decides if you're home' and uncheck the Nest thermostat sensor itself. Manually set a temperature and see if it holds.
IF THAT DIDN'T WORK Check for conflicting schedules. A leftover schedule from Auto-Schedule or a manual entry can still trigger temperature changes. Go to Settings > Reset > Schedule and select 'Reset'. This clears all scheduled setpoints. Then set your desired temperature manually.
HEADS UP If you have multiple Nests, ensure they aren't in a 'Nest Together' group where one's occupancy sensor controls all of them. Also, bright sunlight hitting the Nest's face can trick the sensor into thinking no one is there.