TLDR: Make the 'custom' playlist sort option available on mobile.
Mobile has playlist/library sorting options, while pc doesnt. The ONLY sorting option pc users are used to, the custom order, is not available on mobile anymore.
Sorting order 'custom' for library/playlists to be available on mobile.
Initial default sorting order on mobile: custom (as on desktop).
After changing the sorting order: maintain that choice.
I enjoy organizing my playlists using the desktop spotify app. I still add custom separators between the lists (instead of using folders) using a name like "----------". However, this organization does not carry across to the mobile app.
Contrary to the desktop app, the mobile app offers sorting of playlists, which is weird enough. However, the only order desktop users are used to (the custom order), is not available as a sorting option. Again, this is weird as Spotify does allow you to sort playlists in (mostly*) the same ways as you can on desktop.
*ps: 'duration' is missing as a playlist sorting option on mobile (it is available on desktop). Though I'm not a fan of it, it's strange that it's not available on both platforms.
Additional (system) information:
Desktop version: Spotify for Windows, 184.108.40.2062.g2b11de83
Mobile version: iOS App Store App, 220.127.116.115