Hello!
This is expected behaviour: Horizon is an opinionated theme and we removed the create-topic button from its original place (which, depending on your setup can get rather cluttered), to a new prime location at the top of the sidebar:
On mobile, the sidebar is less easy accessible, so we’re keeping the button there.
This was always the plan, but due to a little error, the topic button made an accidental re-appearance on desktop.