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?
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?