There is no control that submits your work to a topic. Work reaches a topic page through the disciplines you set on it in the editor, and nothing else you type does it reliably. If you want your project on a topic page, set that topic as one of its disciplines and publish.
Disciplines are the route that works
A topic and a discipline are the same thing on UNI seen from two sides. Setting a discipline files your work under that topic, and the topic's page picks it up on its own.
- Projects are listed under the disciplines on the published version of the project.
- Publications are listed under their disciplines, and also under their publication type.
- Journals are listed under their disciplines, and also under their story type.
- Competitions are listed under the disciplines the organizer set, which is not something an entrant controls.
A project holds four disciplines at most, and a loose fourth pulls your work into a stream it does not belong in. See How many disciplines a project can carry.
Tags almost never do it
Tags are free text: you can type anything and it saves. Topics are a catalogue UNI maintains. A tag carries work onto a topic page only where we have attached that specific tag to that specific topic, and the overwhelming majority of tags are attached to none.
So writing a tag does not create a topic, and it does not usually add your work to one. On a project this does not arise at all: the builder has no tag field, and disciplines are the only thing that files a project. Tags apply to journals, publications and your profile.
Topic tabs follow your published version
For a project it is the published version specifically that a topic page reads. Changing disciplines in the builder edits your draft, so the topic page keeps the old set until you publish again. A publication or journal that has slipped back to a draft drops off the topic page until it is published once more.
Give it fifteen minutes
Every tab on a topic page is cached for 15 minutes. Newly published work can take that long to appear, and a discipline you have just removed can linger for the same stretch. Reloading does not shorten it.
Reading a topic page
A topic lives at uni.xyz/topic/its-name, with tabs listing projects, journals, publications, competitions and discussions for that topic alone. Following the topic changes none of it: see What following a topic does.