I did not find an answer to my problem (maybe I was just not looking enough):
If my Samsung S10e is unavailable at the time of closing and saving activity on the Farmin Forerunner 935, the data will not sync when the watch is connected to the phone. When I sync a wired watch to a computer running the Garmin Connect web app, the activity is saved. However, the data does not sync with Garmin Connect Mobile Android and is therefore not available on the mobile phone.
When the phone is connected to the mobile phone, the activities are synchronized in both applications, that sync in the opposite direction works correctly.