The Apple Watch Magnetic Charging Dock MU9F2AM/A is designed with a specific power delivery system tailored to the original cable it comes with, which is a USB-A to Lightning cable. The dock itself is likely optimized for the power output and communication provided through this particular cable.
When you try to use a USB-C to Lightning cable, the power delivery characteristics may differ, leading to compatibility issues. This difference can result from several factors:
1. **Power Delivery Protocols**: USB-C supports different power delivery (PD) protocols compared to USB-A. The dock might not be equipped to handle the PD variations introduced by a USB-C to Lightning cable.
2. **Hardware Compatibility**: The internal hardware of the dock is designed specifically for the included cable. Changing the cable type might affect the way the dock interfaces with the Apple Watch and the power source.
3. **Firmware Limitations**: The dock's firmware might be programmed to recognize and work with the specific power characteristics of the included USB-A to Lightning cable. Switching to a different cable type might not be within its compatibility range.
Given that your Apple Watch Ultra 2 charges correctly with the original cable from the dock and with the charger that came with the watch, but not with the USB-C to Lightning cable on the dock, it indicates that the dock itself has specific requirements or limitations regarding the type of cable used.
### Recommendations:
1. **Use the Original Cable**: To ensure reliable charging, it's best to use the original USB-A to Lightning cable provided with the dock.
2. **Alternative Charging Options**: For faster charging, you might consider using the Apple Watch Magnetic Fast Charger to USB-C Cable that came with your Apple Watch Ultra 2, connected to a USB-C power adapter that supports fast charging.
3. **Dock Compatibility Check**: Ensure that any alternative cables you use are specifically compatible with both the dock and the Apple Watch Ultra 2.
Unfortunately, the design limitations of the Apple Watch Magnetic Charging Dock MU9F2AM/A mean that switching to a USB-C to Lightning cable is not feasible for proper charging of your Apple Watch Ultra 2.
