See
https://www.dokuwiki.org/config:useslash and
https://www.dokuwiki.org/config:sepchar.
Some (bad?) web editors replace spaces in your page names with
%20 and
some browsers will render that as a space (others will fail to decode and deliver the user to a 404 page instead). While most web servers will render a page with
%20, a stock DokuWiki will convert
%20 to an underscore (and even if you create the page outside of DokuWiki it will be ignored).