Same issue here, adding a data point from a different setup: Apple Watch Series 12 (new), watchOS 27.0.1, iPhone on iOS 27 Outdoor runs, recorded with WorkOutDoors (third-party, HKWorkoutSession) [and Apple's Workout app] HR is visible before start. About 30 s after starting the workout, HR turns grey and no samples are recorded for 1–2 minutes, then recording resumes. Happens on nearly every run. Pause/resume sometimes restores HR, sometimes not. An external Bluetooth HR strap works flawlessly every time, so this is not fit, skin or cold. The optical path during workout sessions is not the problem. This did not happen on earlier watchOS versions. It carried over from my previous Ultra 3 to the new Series 12 (both restored from backup), so it doesn't look hardware-related. Since a third-party HKWorkoutSession app is affected too, this supports your theory of a regression in the workout session layer rather than the Workout app itself.