When the Xiaomi Robot Vacuum X20 Max reports a lack of water even though there's still some left, it's because the remaining water in the tank is around 500ml or lower. This early warning is designed to remind you to refill the water tank before it runs completely dry. The intention is to prevent any interruptions in cleaning caused by an empty tank, ensuring that the robot vacuum can continue its tasks without any issues. Refilling the tank promptly will help maintain consistent cleaning performance.