When the Apple Watch Ultra 2 is disconnected from the iPhone, the Heart app cannot measure real-time heart rate

As the title suggests, I'm using the Apple Watch Ultra 2+iPhone 15 Pro Max with system versions of watchOS 10.3.1 and iOS 17.3.1, respectively.

When the Watch is in the following network connection state, it will encounter the problem of using the "Heart" app to display real-time heart rate, as shown in the photo:

The Chinese annotation in Figure 2 means: ECG function is normal and real-time heart rate can be displayed, also in the fitness training app.

1. 1. Normal when connecting to iPhone via Bluetooth.
   2. Abnormal when connect WLAN after disconnecting Bluetooth connection from iPhone.
   3. Abnormal when disconnect WLAN and automatically enable cellular network after disconnecting Bluetooth connection from iPhone.
   4. Normal when disconnecting Bluetooth connection from iPhone and turned on airplane mode.
   5. Normal when manually turning off cellular network after disconnecting Bluetooth connection from iPhone.

I can confirm that there is no hardware issue with the Watch, as I can use other apps (including fitness training apps) to read real-time heart rate, and the "Heart" app also records the latest measurement results and changes in heart rate throughout the day.

I have contacted the technical consultant of Apple Support, and after collecting and analyzing the logs, the response I received is that the problem cannot be solved temporarily and needs to wait for further analysis and processing by the engineering department. I am curious why internet connection is related to the issue of real-time heart rate not being displayed? Looking forward to receiving help from the community.