Sleep Pulse 3 does not directly capture your heart rates. Instead, it retrieves them from Apple Health. Sometimes it could take a few minutes for the data to sync between your iPhone and Apple Watch.
1. Open the Apple Health app on your iPhone
2. Go to "Vitals" > "Heart Rate"
3. Select "Data Sources & Access"
4. Ensure Sleep Pulse 3 is listed as a source. Try toggling off and on the switch to reset the permission.
Additionally, please restart your devices and check if the issue is resolved by swiping right on the main page in Sleep Pulse to "Settings" in the watch app — and tapping "Sync Again" to re-analyze your data.