As with the 5-series, there is a bug with alarm set to vibration only.
The software ignores this setting and activates sound alarm in parallel to vibration.
On my watch i observe this issue with the 5th alarm set - the first 4 alarm are working (yet) correctly.
I thought that Gamin can at least make a simple alarm reliably working.
However even in this point Garmin is not able to make it right.