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.