I have exactly the same problem. Access permissions on folders are ok. I can copy a file from the iCloud drive into my documents folder , edit there and move it back to the iCloud drive. So it looks like it has something to do with files/folders permissions for applications using the iCloud drive. Could not find a way to add the numbers app to the "Files and Folder" option under security and privacy, just to test if that would help solving the problem. Im using Catalina version of macOS too , all patch versions applied.