The Nextcloud-Client is one of my auto start applications. On Plasma-Wayland it starts automatically as a Window but I would expect that will be shown only in the tray, like on X11. Is this a bug in Kwin, Plasma or is the application responsible for starting to tray? For reference, I found an open bug for Nextcloud-Client https://github.com/nextcloud/client/issues/99.
That's the task of the application