Filed as FB24834050. Still reproducible on watchOS 27.0.1 (24R365).
Posting here in case an Apple engineer can help route it, and to see whether other developers are seeing the same behavior through HealthKit.
Minimal reproduction (under 3 minutes, no distance involved)
Apple Watch Series 10 (GPS) running watchOS 27.0.1 (24R365), paired to an iPhone 15 running iOS 27. Start an Other workout. Move your arms continuously for about 2 minutes (I use air punches). No treadmill, GPS, or distance calculation involved. End the workout and review the heart rate samples.
Result: One heart rate sample at approximately 0:05, then no additional samples for the rest of the workout.
Expected: Continuous heart rate sampling every few seconds, which is how the same watch behaved on watchOS 26.
This reproduces 100% of the time across every workout type I've tried since updating to watchOS 27.0. It did not occur on watchOS 26 using the same watch, same wrist, and same fit.
Key diagnostic: the sensor is working, the workout session path is not.
The Heart Rate app reads immediately and continues updating every few seconds, even while a workout is running. Background heart rate sampling outside workout sessions works normally. Active energy for my daily 65-minute session is accurate when no workout is started (405 calories on 9/22/26, consistent with watchOS 26). Recording that same session as an Indoor Walk workout results in roughly half the active calories (typically 185-200 versus high-300s/low-400s before the update).
Starting a workout appears to suppress a working measurement path and replace it with one that is failing.
Downstream impact
A 1:06:13 session has been credited as 9, 35, and 55 exercise minutes on different days. Since exercise-minute credit for walking is heart-rate driven, the lack of heart rate samples appears to directly affect exercise credit.
Separate issue in the same build (not the subject of this post)
Indoor distance is consistently about 50% low. A 1:06:13 walk at a constant 4.1 mph should be 4.52 miles, but the watch reports roughly 2-3 miles even though step counts are correct.
The watchOS 27 User Guide describes a redesigned pedometer model using new machine-learning algorithms for treadmill distance. Series 12 and Ultra 4 owners are reporting the same shortfall with matching step counts, including on clean installs, so this does not appear to be hardware-specific.
The heart rate dropout reproduces in the Other workout type, which performs no distance calculation, so the two issues appear to be independent regressions.
Already ruled out
Health details are correct Wrist Detection enabled Power Saving Mode off on both watch and iPhone Heart Rate and Fitness Tracking permissions enabled Motion Calibration & Distance enabled Watch fit and position unchanged Both devices restarted Fitness Calibration Data reset Full unpair/re-pair Multiple workout types tested Pause/resume mid-workout (heart rate briefly shows "Searching" and then drops out again)
FB24834050 includes two sysdiagnoses from watchOS 27.0 and currently shows "Recent Similar Reports: More than 10." Status remains Open with no response.
Ask
Can anyone from Apple confirm whether this is a known/tracked issue?
Also, would a sysdiagnose captured using the watchOS logging profile from the Apple bug-reporting site be useful? I'm happy to install the profile, reproduce the issue in under three minutes, and attach fresh logs.
I'm also interested in hearing from developers using HKWorkoutSession. Are you seeing the same heart-rate sample gaps on watchOS 27 in your own apps? That would help determine whether this is specific to the Workout app or a regression in the workout session layer underneath it.
Thanks!