Everything Carl has just written aligns with the responses I had to this issue from the Fenix beta folks at Garmin. The only thing I'd add is that it's a change in the Garmin Connect website/ecosystem that you'll need to keep an eye out for, rather than the Garmin Connect Mobile app (settings/alarms are overwritten when syncing via Garmin Express or direct wifi, so it's the whole Garmin Connect ecosystem that needs to be fixed).
If I had to choose between settings alarms on the watch and on GCM, I'd choose the watch any day. Having the function in GCM and letting it confusingly overwrite the alarms you set in the watch is super confusing, and when you're camping and have spotty service, it's even more so, because you can't even load the app most of the time. But maybe it will load and change your alarm settings while you're sleeping, and cause all kinds of problems. I ended up putting my phone in airplane mode before a triathlon just so my Fenix 5 wouldn't connect to GCM and mess up my alarms before the big race. Terrible bug, if it can even be called that.
Just simplify and make alarms watch-only. Works great.
This is a bug, not a bad design as neither the Fenix 3 nor the Fenix 3 HR had this issue, it is really bad that isn't acknowledged.