Hi,
On this thread we've had an issue where Garmin Express after sync deleted all activities. I've confirmed it on my side by downloading manually my activities from GC web and putting them back on the watch. They were gone again after sync via Garmin Express. I did the update to latest version of Garmin Express today and it is with the latest version that I had this issue.
Here is the link to original thread in Fenix 6 forum.