It works when I type in my name...
Bug in clubs end point

Actually, I think their account was closed, either by themselves or by a moderator/staff. Closed and banned accounts don't show up when you search them, but if you're friends with them, you can still veiw their profile. Kind of strange...

For the website, https://www.chess.com/member/skelos (substitute username of interest) also works for closed accounts, no matter the closure reason. It won't work for a closed and renamed account of course.
Different result from web page and API in member club end point.
https://www.chess.com/clubs/home/gutsygecko
Shows membership of two clubs
https://api.chess.com/pub/player/gutsygecko/clubs
Returns