Thread B is subscribed by users x,y,z
If I merge both threads to Thread A, from my understanding only users a,b,c are still subscribers.
Subscribers x,y,z are removed as subscribers from deleted Thread B and not added as subscribers to Thread A
Solution: XenForo_Model_Thread::mergeThreads() can be modified to fetch subscribers from both threads and then add the missing ones from the deleted threads to the destination thread.
From my question http://xenforo.com/community/threads/whos-getting-alerts-with-merged-threads.57650/