I’m afraid this isn’t a bug, and is working as intended. Creating an Unlisted topic removes the topic from topic and search lists, but will still send a notification to anyone Watching/Watching First Post for the category (or tag).
I believe the advice for creating a topic you don’t want people to see in advance is to use a permission-controlled category (eg staff) and to manually recategorise it when ready, or Schedule Publishing from the topic timer. You can also use the Shared Draft feature as well.
This issue does crop up reasonably regularly though, so may worth collating into a feature topic.