@stefan-kersting so while the activity is going on the watch adds data to the “activity file”, such as elevation, coordinates, HR, etc. At the end it computes some statistics and adds them to a header.
What I understand is that in case the watch experience a crash, you end up with empty or incomplete header. Now SA and other applications can compute some stuff on their own, but for some things they rely on the header. This is why you have some data but not all of them.