I was unable to find any report covering this doing a cursory search. When a thread is merged with a bigger thread on your watch-list, shouldn't that be flagged as new posts and trigger an alert? Especially if you have not read the thread being merged? An example would be the California Case Update thread and the threads created when there is a PACER update. As far as I can see, I only got alerts when the larger thread was reopened and got new replies.