When creating a Topic with Auto-Follow enabled, it is still prompting users to follow the channel instead of this happening automatically.
Studio - Topics
It's currently expected that when you create a new topic (formerly channel) without a target audience so that all registered users have access to the topic, all registered users would be auto followed to the topic. However right now no one is auto followed.
This issue has now been resolved and moved to the Completed Issues Tracker. (Issue ID: PUB-1894)