Yes, this is correct. It will work fine sending out e-mails and then all of a sudden after maybe a day or two it stops sending out the e-mails. All of the alarms still show up in the alarm tables and get saved into the ALERT_LOG database table but the e-mails don't go out. After a gateway restart it will work again for a few days and then stop.
This started happening after the upgrade from 7.5.0 to 7.5.1 if that helps at all. E-mails worked without a problem for me from 7.2.5 (my original install) to 7.5.0.
We do not have a failover database, however our database is in a MSSQL cluster on virtual servers. But still, there is only "one" database that Ignition connects to for alert data.