If you notice bubbles in the water tank of the Xiaomi Robot Vacuum X20 Max, this is due to a protective film applied during the manufacturing process. This film is used to prevent scratches or damage to the water tank's exterior during production and shipping. As a result, some small air bubbles will become trapped under the film, but this is not a defect or an issue with the tank's appearance. The bubbles are simply a byproduct of the protective measure.
You can choose to remove the protective film if you prefer a smoother appearance, or you can leave it on and continue using the robot vacuum as it does not affect the device's functionality.