Apple Watch SE workout or activity was not recorded correctly

When a workout vanishes from the Apple Watch SE (2nd generation), the cause is nearly always that it never finished saving or never reached the iPhone, not that it was deleted. A workout must be ended properly: pressing the side button and the Digital Crown pauses it, and the workout is saved when you swipe and tap End. If the watch ran out of battery or restarted mid-workout, the data may be lost. Open the Fitness app on the iPhone and the Health app, since a workout can appear in one before the other. Make sure the watch is paired and syncing with the iPhone, Bluetooth is on and the Health permissions for Fitness are allowed. If data is still missing, restart both devices and wait for a sync.

Third-party workout apps save to Health only if permissions are on. Apple Watch workouts are stored first on the watch and sent to the iPhone through the Health app, so if you changed iPhones or reset the Health data, old workouts may be absent from the new device until the backup is restored. Check Health, Browse, Activity, Workouts for a list of everything the iPhone holds. If you use a third-party running or cycling app, check whether it saves to Apple Health and whether the watch's own Workout app was used at the same time, because two apps recording one session can produce a duplicate or a missing entry. Use one app per session and make sure the other is closed.