The Xiaomi 15 has an IP68 protection rating, but since the internal structure is not vacuum-sealed, it cannot completely isolate air. In certain conditions, water vapor from the air can enter the phone. When there is a sudden drop in external temperature or a significant temperature difference between the inside and outside of the phone, condensation can form on the camera lens, causing fogging. This is a normal physical phenomenon, similar to how eyeglasses fog up when transitioning from a cold outdoor environment to a warm indoor space. There is no need for concern, as the fog will naturally dissipate after being placed in a stable environment for some time (e.g., after a few days of use, the internal moisture will evaporate).
To speed up the process, you can remove the SIM card tray and place the phone in a sealed, dry environment, such as a sealed bag with a desiccant. If you need to use the camera immediately, you can try warming the lens by touching it with your fingers or palm to help the condensation dissipate more quickly.