When updating an existing site to 1.4.0i2 or 1.4.0i2p2 while not having the "Check_MK notification spooler" enabled via "omd config", the notification system stopped sending notifications.
The background is that the default notification mechanism had been changed from direct delivery to asynchronous delivery. And this was done even when the notification spooler, which is responsible for delivering the asynchronous notification, was disabled.
The notifications are not lost, they have simply not been processed. In case you are affected, there should exist one file for each notification that has been created but was not delivered below var/check_mk/notify/spool/.
To fix this issue just install the new version. After the update all notification will be sent out. In case you want to supress the stuck notifications, you can go to the directory mentioned above and remove the .mk files in this directory before starting the site again.
To make it work for you with the existing versions, you can execute "omd config" and go to "Distributed Monitoring > MKNOTIFYD" and enable this option. After starting the site again, all your notifications will be processed and are sent to the users.