Why is bpm -1 in Apple heart rate samples?

A bpm value of -1 typically originates from the source data in Apple Health, not from Terra processing.

A bpm value of -1 typically originates from the source data in Apple Health, not from Terra processing. Terra forwards what the health source provides.

To check, open the Apple Health app's 'all data' view for those timestamps. A third-party app may be writing -1 directly into HealthKit.