NFS4.2 have support to synthetic backup, but it is necessary to use XFS under NFS share. NFS only provide possibility to use synthetic file system under share, we support XFS as Synthetic FS.
If you still have this problem, please send us the full DEBUG logs of this operation (server and node logs). If you cannot attach logs to this thread, you can put them somewhere, e.g. OneDrive or similar, and send the link.
Here is the documentation on how to switch logs to debug: Logs in DEBUG