When using the Task Manager with Wayland, the Filters are not applied correctly: The 'Show only tasks from the current desktop' Filter is never activated, even when enabled in the settings. Only when enabling 'Show only tasks from the current screen' the tasks from different Desktops are not shown, I assume since the screens from different desktops are always considered to be different.
Virtual desktop related functionality is currently not working on Wayland, see: https://community.kde.org/Plasma/5.11_Errata
Okay, thanks for the update. I was not aware of this.