Thank you for reporting this issue. We couldn't find anything in how merges work that would stop an alert from adding to the alert badge count. Watched-thread alerts are deliberately held back while the thread has posts you haven't read, and a merge can add such posts. We need a little more detail to look into this.
How did User X see the alert without the badge: in the alerts menu, on the alerts page, through a push notification, or by email? Was the badge still missing after a full page reload?
Before the merge, had User X read thread B (the target), or only thread A? Did User X open the merged thread at any point between the merge and the replies?
Was User X also the moderator who did the merge, or using the same browser session as that moderator?
In the second test, did User X visit the merged thread before the later posts that did alert normally?
Are any add-ons installed that change alerts, thread watching or merging? Does this happen with them disabled?
This is an automated assessment. A member of staff may review and update it.
