For a Channel Site that's created for a Team (in this instance, a "Shared" channel) within MS Teams, a SharePoint Channel Site is also created along with a Shared Documents document library for that new channel.
But the problems begin, when the user then creates multiple Document Libraries for that Channel Site, SharePoint fails to display these Document Libraries within many of the File Operation menus, which causes complications when users are attempting to Copy/Move files and folders to that Channel Sites custom Document Libraries.
The workaround involves the user firstly having to move their documents to the Channels default "Shared Documents" Document Library, then navigate to that location, and THEN move their documents again to their final destination as it's from this point only will SharePoint show them the location of their desired custom created Document Libraries nested within that Channel site.
This is a very painful experience for the user, and needs to be addressed asap. Often, especially with a Hub site, users may need to move files from one Channel to another, and not being able to see target custom Document Libraries is a huge fault (at least in my mind).
Edit: Here is an image that shows the missing Document Library navigation option that users are expecting when navigating Channel sites. There's simply no way to display/navigate/copy files into these Document Libraries when navigating from anywhere (such as a Hub site, or Onedrive folder) - the only exception being, they display at least when navigating from the Channel site itself.