For your reference, the reasons for this issue are as follows:
1. We offer a wearing mode specifically designed for necklace pendants. In this mode, the device functions primarily as a pendant, enhancing its aesthetic appeal and presentation. As a result, it only supports step count detection and does not track other exercise or health-related data items, such as calories burned or specific workout metrics;
2. When worn as a necklace, the sensors on the Xiaomi Smart Band 9 are not in the optimal position to accurately capture the various physical activities and heart rate data required for comprehensive exercise tracking. This positional change impacts the device's ability to monitor workouts accurately;
3. The Mi Fitness APP provides detailed information on the capabilities and limitations of the necklace mode. Within the APP, people can access the usage guide ([Mi Fitness] APP >> [Device] >> [Help center] >> [Usage guide]), which notes the specific data items that can be monitored in necklace mode and explains the special logic applied when this mode is enabled. For more detailed information and to ensure you are making the most out of your device, please refer to the guide.