The car dashboard indicates that the water temperature is high, but the heater is never hot. After turning off the car, it feels like the water tank fan is still running and needs to wait for a while before stopping.

This is a new car. If this happens to a new car, it is a quality problem of the product, rather than something caused by it. According to your description, it should be because the engine water temperature is too high and the general cycle is turned on. Normally this will not be the case, as there should be no hot water flowing through the warm air return pipe. Generally, this situation occurs more often. If you just bought a new car, you should go to the 4S store to check it out. If there is any defect, just replace it without repairing it. This is free of charge during their warranty period.