Did you ever get a fix on this? I have a long thread here about the issue on my Fenix 6. Started having this issue in June. Factory reset in October and no issues for a month but I'm now back to three corrupt swims in a row. But at least todays swim synced through as a multisport event so I actually have some data!
Garmin support were helpful and asked me to send them files to help them diagnose the issue. The problem continued through a number of software updates but eventually they released an update that fixed it. Do you know if the problem recurred after a software update?