A user, caiiiycuk, is experiencing an issue where visiting a topic URL without an ID (using slug only) redirects to the welcome topic, but works normally when clicked from another topic. Falco explains that this is the intended behavior, as Discourse treats URLs starting with a number as IDs. pfaffman advises against using slug-only URLs, as they can break if the topic is renamed and are more work to use than URLs with IDs.
That’s more work than using the url in your browser (that has the ID) and will break if you rename the topic. (plus when the title starts with a number)