When a file that's stored in iCloud gets moved out of local storage (automatically by the OS I assume), as indicated by the dotted cloud symbol, and then automatically restored (again by the OS I assume) when it is used, it loses the remote link information.
For example, I have a folder of files that is sourced from a SSH connection, stored in iCloud. All works fine, until iOS decides to clear space by removing some of these. They get restored automatically, so that's fine, but once that's done I can no longer upload/download because the link is gone.
Customer support service by UserEcho