In the developer console, reenabling notifications for one alert also reenables the notifications for other alerts. In other words, trying to expire one silence actually expires other silences too.

4.15, 4.14, 4.13, possibly earlier, but not tested yet

Always

1. Navigate to Dev console > Observe > Alerts page
2. Disable notifications for 2 different alerts
3. Reenable notifications for 1 alert
4. If testing with 4.13 or earlier you will probably need to refresh the page

Both of the alerts now have notifications enabled

Only the one alert that you reenabled notifications for should have changed. The other alert should still have notifications disabled.

Disabling / reenabling notifications is actually creating / expiring Alertmanager silences, so what is happening is that when notifications are reenabled for one alert, both of the silences are being expired. Only the one silence for that alert should be expired.