That's why I concocted an unofficial alternative version.
The problem is that the real name of a free-space page starts with Space:, so from the point of view of the A-Z and the Previous/Next buttons, they're all lumped together under S, even though they don't display that way.
Basically I think the MediaWiki code is broken, or strangely designed. No large categories sort and page properly, even if they only contain ordinary profile pages.