A very common cause of this kind of issue is that syncing Temp sensors to the thermostat must be added to the schedules themselves for it to remain connected when using a schedule. It looks like one thermostat is using a temp sensor on its schedule but the “Upstairs” one does not. Have you left the sensor disconnected and deleted it from the schedule intentionally or do the schedule selections not save?
Often the issue is that the temp sensor is connected on the main card, which is used when not running a schedule, but not on the schedule.
To confirm, you would need to sync it on the main card, then go to the thermostats page in Alarm.com, select Schedules, then select the thermostat you want to edit the schedules from the select box in the upper left.
On each applicable schedule period, hover over the name and click the edit pencil, you can then add the Temp sensor to those schedule periods.
If the steps above are performed and it is still dropping from the Tstat, I would first delete that RTS from the system, wait about 5 minutes, then learn it back in. Be sure it is in its permanent location when learned. After, does its behavior change when connected to the schedule?