Files and Folders in 'Documents' in iCloud not showing on iMac 24 (M1) or iCloud

OK, so I haven't found why it happened but I have a solution. Checked what file provider knew (on Mac and MacBook) via 'fileproviderctl domain list' and these were pretty much identical listing iCloud and OneDrive. One difference was initially the Mac version showed '(not-authenticated)' against iCloud but after removing the FileProvider folder and killing Finder ('killall Finder') to recreate it the two were then identical. This didn't solve the problem. Also tried re-installing Sonoma - didn't solve it. Finally decided to back up iCloud to local ext HD and logged out of iCloud on Mac (in Settings). Hoping this wouldn't lose any files when iCloud re-synced - i was concerned iCloud may decide there were no files locally and removed them all from the Cloud when re-syncing. Luckily this worked. Logged back into iCloud (in Settings) and checked sync 'Desktop & Documents Folders' and are all now visible (after a short wait to re-sync). So success, but don't know what caused iCloud to hide the folders and how it could re-hide after un-hiding! Any info on whether there's a controlling file that tells iCloud/fileprovider what to do would be interesting....

After some more digging it appears that something has hidden the folders. I can copy a file to iCloud/Documents and it briefly appears then disappears. Copying again gets the helpful message that the previous file is hidden! I can unhide the files and folders with chflags but mysteriously as soon as this is completed something comes along and hides them again (prob via the fileproviderd when I check the process table). I see the files disappear in front of me. Initially thought it may be BitDefender as that ‘protected’ some of the iCloud folders, but I’ve removed the folders from its list and no change. Plus, this list has been in place for months/years (and is also on MacBook that doesn't have this issue). Still digging...