Its not missing from the site but listed as a game and not an app so it dosent show on the apps list but instead the games list. This is a peice of software for wallpapers and is listed as software on steam also.
Wallpaper Engine
https://store.steampowered.com/app/431960/Wallpaper_Engine/Also I have a game that is listed as an app so it has the reverse problem.
SoulWorker - Anime Action MMO
https://store.steampowered.com/app/630100/SoulWorker__Anime_...