I got the same thing and it happened because I was watching the thread that post was originally in (I foget which one) and then it got split from the thread into it's own thread.
Didn't realize that was split out, but if so, then that is generally the expected behavior. Of course, it comes down to intentions, but we don't know why you watched a thread, so it posts are moved out, it makes sense to watch the new thread.
If posts are split and placed in a "trash" forum (for moderator review for example), will that new thread be watched as well even if the members don't have permission to view the forum it's been split into?
I don't know if it was part of a watched thread. If it was, then I believe the newly formed thread should be regarded as a completely new thread with no watching attached to it. And it is because the new thread shows to me as not watched. So why receive alerts for a thread that is not watched?
Hmm, actually there are no watch records for the new thread. I think you got an alert for that post when it was in the thread. It then got moved, but you still had the alert.
The only real option is to remove alerts when we move posts out of a thread, but I think that's a bit blunt. It should be pretty rare, so I'm not going to make a chance for this now. We'll see if it comes up over time.
OT I know, but does vB 3.8x work like that too - always wondered that when I post updates to threads moved to a mod-only forum. I presume, as with XF, it doesn't notify people about the new post if they can't view the thread?