DIAGNOSIS An active 'Away' schedule or geofencing feature in the Sensi mobile app is overriding setting.
FIX Open the Sensi mobile app. Tap on the thermostat, then tap the Schedule icon (calendar). Check if an 'Away' period is currently active and tap 'Cancel Away'. Go to Menu > Settings > Geofencing and ensure it is disabled unless you intend to use it.
IF THAT DIDN'T WORK Check for a conflicting schedule. In the app, go to Menu > Schedule. Review the scheduled periods for each day. A schedule event with the system mode set to 'Off' will override a manual 'Heat' setting when its time arrives.
HEADS UP If you have multiple users with app access, another person's 'Away' setting or schedule change can affect the thermostat. Verify all user accounts and their settings.