I’m not sure if this is a bug or a UX issue. It puzzled me on my forum, and I can reproduce it on try.discourse.org.
What happens is that if you select “no tags” in the tag selector on a topic list, and then you try to create a new topic, the tag field in the composer is automatically pre-filled with a tag named “none”. Even if you’re an administrator, it also prevents posting the topic because of a “Tag is Invalid” error message.