I found the following statement in the „File System Programming Guide“:
„Apps create files and directories in iCloud container directories in exactly the same way as they create local files and directories. And all the file’s attributes are saved, if they add extended attributes to a file, those attributes are copied to iCloud and to the user's other devices too.“
Obviously that is not the case as I found my attribute as well as „com.apple.ResourceFork“ being stripped during iCloud synching to other devices. Other developers are observing the same behavior, here is a well-describing article about the problem:
I've invested a DTS credit, but so far haven't received a response...
This is just too sad: I filed the radar over two months ago, it still is in status „open“ with zip reaction from Apple. Unfortunately I have to take some serious decisions about the app‘s future development based on the chances / timeline to get this issue resolved. I asked for some response either via bug reporter and Developer Support ... but still no update.
I think I will file a new TSI just to get any update. Sorry, Apple, that‘s no way to support an active developer.