That's normal operation. The Warm function has a 10-minute automatic shut-off.
It's a safety feature to prevent unattended operation. The function is designed to hold cooked food at a serving temperature for a short period, not for prolonged warming.
If you need to keep food warm longer, you must manually reactivate the Warm function. Press the Power button to turn the unit off, then select Warm again. The display will show the 30-minute countdown from the beginning.
Heads up: Do not use the Warm function to reheat cold food or cook food. It maintains temperature only. Always place cooked food on the provided baking pan or in an oven-safe dish when using Warm.