This is actually intended: As long as Syncthing is actively handling a temporary file, it is hidden. The user should not interact with it, as it will be used by Syncthing and subsequently vanish again. If the operation including this file is aborted (Syncthing stops, remote vanished, sync paused, …) Syncthing keeps the temporary file (afaik for 24h) such that it can be reused if the same operation is done again soon. In case Syncthing stopped, it is no longer temporary (or at least on a much longer timescale), so it makes sense for the user to be aware of them (and e.g. remove them for space or something).