User shares are how Unraid allows folders to span disks.
When you create a user share, Unraid creates a top level folder named for the share on cache or array as needed in accordance with the settings for the user share.
Conversely, any top level folder created on cache or array is part of a user share named for the folder. If you don't make settings for a share it has default settings.
Cache is part of the user shares, and each array disk is part of user shares. If a top level folder exists on cache, and a top level folder with the same name exists on an array disk, then those folders are part of the same user share.
You might try deleting the appdata for plex in addition to deleting the docker and reinstalling plex from scratch, but that would mean you would be starting over with plex and would have to rescan all your media.