There's a post on the Garmin Connect forum about this, but it's from a 5x user and I'm not sure it's a GC issue and instead might be a Fenix issue. But it happened to me, too. Basically a couple days ago, for no apparent reason, the setting to for my watch to be my preferred activity tracker was lost. I didn't change it, but it went away. Now I'm missing two days of activity data. Not the end of the world, but frustrating.
Basically I opened GC and on my daily report there was no HR, no SPO2, nothing interesting. And there hadn't been for a couple days. My watch was giving me notifications and was syncing workout data on those days, so until I opened GC I had no idea this had changed. Once I changed that setting, it did a HUGE sync (ie. that next sync took like 20 minutes, maybe more) and I assumed I'd get those lost days. But no, it was not to be. I don't know what the heck it was sync'ing for that long (it was previously synced just fine, it was NOT workout data).
Anyway, word of warning.
--Donnie