Steps to reproduce:

1. Create two forums ('Maths', 'General chit chat') in a course
2. As student1, subscribe to the posts in 'Maths' but not 'General chit chat'
3. As student2, post about a joke in the Maths forum
4. As teacher1 move the post from 'Maths' to 'General chit chat' before the end of the editing window (as its offtopic for the study forum)

Expected result:

• Student2 doesn't receive copies of student1's forum post which is offtopic

Actual result:

• Student2 receives copies of the off-topic post

Further discussion:

Moving the subscriptions before the editing window is over (and thus, posts have gone out) is really confusing to the user, because they will get notifications about forum posts in a forum they are not subscribed to without any indication of why it happened. But even after the editing window is over - a frequent reason for forum post moves will be done as a moderating action to prevent offtopic discussions from carrying out in the wrong forum. I do not think it makes sense to retain 'forum' subscribers (rather than thread subscribers) in many scenarios