Fitbit Sense 2 workout or activity was not recorded correctly

The checks in more detail

on Android, Nearby devices permission and battery optimization for the app matter, and on iPhone, Background App Refresh does. A force-closed app does not sync until reopened. Look in the app under the Sense 2's settings for Sync now, and keep the watch near the phone for a minute. If the watch has no record, the exercise was not recorded. The Sense 2 has automatic exercise recognition, which detects walks, runs and some other activities after about fifteen minutes, but short sessions and unusual movements go unnoticed;

for a precise record, start the activity from the Exercise app. The watch has GPS built in, so a run needs a moment for a fix, and a poor fix can fail to save a route; wait for the signal indicator before you start. If the watch was low on battery or restarted during the session, the record may be lost. Check that you pressed finish or hold to end properly, because a session left running can be dropped. If the workout was recorded and synced but is missing from your list, check the date range and filters in the app's Exercise log, and the Fitbit account you are signed in to.

Update the watch software and the app, which Fitbit does through the app. A watch worn too loosely may lose the heart rate and mark parts of a session as missing. Restart the watch and the phone. If workouts still vanish, remove the watch from the app and set it up again after a sync, which keeps your data in the account, and contact Fitbit if it keeps happening. Keep a note of the date and exercise type for support.