I kind of see this as a feature request:
You are about to change categories on 1293 topics which will notify 8000 people watching the #new category, Would you like to notify them?
There is also the possible new feature of a “bulk” notification for cases like this that would need to be fleshed out:
784 topics changed from categoryA to category you are watching / watching first post.
Does not feel like a bug, but more like a possible improvement we can make.