First, imagine that you have an User with the slug “lukas”.
When visitor types www.yoursite.com/lukas Bubble will not find the page “lukas”, so it will redirect the visitor imediatly to the 404 page. The information that your visitor is in the 404 page will not appear anywhere and your URL will remain the same as before.
In your 404 page just run a workflow “On page load” to Search for User’s where Slug is “lukas”. Done. You have your Users information and you can create his/her profile without problem.
The downside of this procedure is the redirection time (2 or 3 second) and the SEO. If they were not a problem, you can use this method.