Diagnosis
The thermostat is losing power briefly or experiencing a software glitch that corrupts the schedule memory.
How to Fix It
Check for intermittent C-wire power. Turn off furnace power. Inspect the C wire connection at the furnace control board. It must be tight. Also check for a loose wire nut if your thermostat cable was spliced somewhere. A flickering connection can cause a reset.
If That Didn't Work
Perform a full reset and reconfigure. Menu > Settings > Reset > Full Reset. Set up Wi-Fi and your schedule again from scratch. This clears any corrupted memory that a soft reset doesn't fix.
Heads Up
If your furnace has a flame rollout or overheat safety switch that trips often, it can cut 24V power to the thermostat, causing a reset. Frequent resets point to an underlying furnace problem.