This sounds like something either Deluge or the Deluge API plugin should fix.
We have no way of knowing it it will be set, its a separate call after the torrent has been added which is pretty common for Torrent clients it seems. We’re not going to start babysitting deluge to see if its doing whats its been told to, the problem with force resetting the label is we would need a task to periodically check and if the label was intentionally changed Sonarr would forcibly set it again, which seems wrong.